Fix PreallocatedMap::find to check for missing keys.
Fix PreallocatedMap::find to check for missing keys.

This is a quick fix for a hang in EnumSimplification.

Swift SVN r20987

milseman/swift,harlanhaskins/swift,ahoppen/swift,airspeedswift/swift,adrfer/swift,tjw/swift,MukeshKumarS/Swift,rudkx/swift,sschiau/swift,LeoShimonaka/swift,austinzheng/swift,sschiau/swift,roambotics/swift,jckarter/swift,mightydeveloper/swift,tinysun212/swift-windows,LeoShimonaka/swift,CodaFi/swift,practicalswift/swift,practicalswift/swift,xedin/swift,benlangmuir/swift,austinzheng/swift,emilstahl/swift,milseman/swift,shahmishal/swift,austinzheng/swift,bitjammer/swift,milseman/swift,tkremenek/swift,modocache/swift,bitjammer/swift,modocache/swift,gregomni/swift,MukeshKumarS/Swift,zisko/swift,danielmartin/swift,nathawes/swift,milseman/swift,OscarSwanros/swift,apple/swift,sschiau/swift,jckarter/swift,devincoughlin/swift,swiftix/swift.old,nathawes/swift,russbishop/swift,gmilos/swift,calebd/swift,calebd/swift,Jnosh/swift,alblue/swift,johnno1962d/swift,tkremenek/swift,atrick/swift,atrick/swift,amraboelela/swift,bitjammer/swift,parkera/swift,gottesmm/swift,ben-ng/swift,zisko/swift,mightydeveloper/swift,atrick/swift,kusl/swift,gottesmm/swift,ben-ng/swift,jopamer/swift,jckarter/swift,amraboelela/swift,sdulal/swift,swiftix/swift,karwa/swift,harlanhaskins/swift,allevato/swift,cbrentharris/swift,tardieu/swift,jtbandes/swift,JaSpa/swift,karwa/swift,MukeshKumarS/Swift,tjw/swift,danielmartin/swift,arvedviehweger/swift,xedin/swift,russbishop/swift,therealbnut/swift,tkremenek/swift,zisko/swift,amraboelela/swift,aschwaighofer/swift,codestergit/swift,xwu/swift,hooman/swift,sschiau/swift,danielmartin/swift,swiftix/swift.old,IngmarStein/swift,milseman/swift,ken0nek/swift,jtbandes/swift,dduan/swift,gmilos/swift,tardieu/swift,allevato/swift,benlangmuir/swift,alblue/swift,jtbandes/swift,return/swift,JGiola/swift,gottesmm/swift,sdulal/swift,xedin/swift,IngmarStein/swift,deyton/swift,Ivacker/swift,xwu/swift,swiftix/swift,OscarSwanros/swift,parkera/swift,kstaring/swift,arvedviehweger/swift,IngmarStein/swift,cbrentharris/swift,kperryua/swift,russbishop/swift,xedin/swift,emilstahl/swift,LeoShimonaka/swift,atrick/swift,manavgabhawala/swift,uasys/swift,Ivacker/swift,tardieu/swift,benlangmuir/swift,xedin/swift,lorentey/swift,benlangmuir/swift,hooman/swift,kperryua/swift,milseman/swift,deyton/swift,airspeedswift/swift,rudkx/swift,devincoughlin/swift,airspeedswift/swift,amraboelela/swift,glessard/swift,jtbandes/swift,JaSpa/swift,harlanhaskins/swift,tardieu/swift,deyton/swift,huonw/swift,parkera/swift,JaSpa/swift,sschiau/swift,cbrentharris/swift,huonw/swift,airspeedswift/swift,felix91gr/swift,natecook1000/swift,felix91gr/swift,modocache/swift,manavgabhawala/swift,codestergit/swift,alblue/swift,cbrentharris/swift,alblue/swift,JGiola/swift,hughbe/swift,practicalswift/swift,russbishop/swift,tinysun212/swift-windows,felix91gr/swift,uasys/swift,mightydeveloper/swift,slavapestov/swift,xwu/swift,jmgc/swift,brentdax/swift,return/swift,therealbnut/swift,IngmarStein/swift,huonw/swift,sdulal/swift,calebd/swift,slavapestov/swift,frootloops/swift,apple/swift,djwbrown/swift,apple/swift,JGiola/swift,natecook1000/swift,Ivacker/swift,rudkx/swift,shajrawi/swift,brentdax/swift,kusl/swift,kentya6/swift,amraboelela/swift,jmgc/swift,glessard/swift,lorentey/swift,devincoughlin/swift,alblue/swift,kstaring/swift,tjw/swift,ben-ng/swift,kstaring/swift,kusl/swift,manavgabhawala/swift,emilstahl/swift,kentya6/swift,OscarSwanros/swift,devincoughlin/swift,practicalswift/swift,danielmartin/swift,ken0nek/swift,swiftix/swift,Jnosh/swift,return/swift,hooman/swift,djwbrown/swift,shajrawi/swift,stephentyrone/swift,JGiola/swift,glessard/swift,manavgabhawala/swift,stephentyrone/swift,kusl/swift,hooman/swift,harlanhaskins/swift,slavapestov/swift,kstaring/swift,frootloops/swift,glessard/swift,xedin/swift,jmgc/swift,Jnosh/swift,benlangmuir/swift,Jnosh/swift,hughbe/swift,tkremenek/swift,codestergit/swift,hughbe/swift,frootloops/swift,JaSpa/swift,tinysun212/swift-windows,jckarter/swift,brentdax/swift,jtbandes/swift,sdulal/swift,apple/swift,slavapestov/swift,gribozavr/swift,JaSpa/swift,CodaFi/swift,ben-ng/swift,Jnosh/swift,lorentey/swift,CodaFi/swift,russbishop/swift,gottesmm/swift,swiftix/swift,return/swift,gmilos/swift,adrfer/swift,manavgabhawala/swift,tkremenek/swift,cbrentharris/swift,therealbnut/swift,shahmishal/swift,nathawes/swift,tardieu/swift,shahmishal/swift,gribozavr/swift,tinysun212/swift-windows,shajrawi/swift,rudkx/swift,LeoShimonaka/swift,practicalswift/swift,allevato/swift,alblue/swift,parkera/swift,gmilos/swift,airspeedswift/swift,parkera/swift,ben-ng/swift,calebd/swift,frootloops/swift,SwiftAndroid/swift,arvedviehweger/swift,jtbandes/swift,emilstahl/swift,gribozavr/swift,gregomni/swift,glessard/swift,danielmartin/swift,jmgc/swift,natecook1000/swift,gribozavr/swift,deyton/swift,lorentey/swift,emilstahl/swift,deyton/swift,KrishMunot/swift,kstaring/swift,tkremenek/swift,johnno1962d/swift,kusl/swift,mightydeveloper/swift,karwa/swift,kentya6/swift,mightydeveloper/swift,shajrawi/swift,JGiola/swift,austinzheng/swift,tkremenek/swift,Ivacker/swift,uasys/swift,shahmishal/swift,modocache/swift,nathawes/swift,felix91gr/swift,mightydeveloper/swift,roambotics/swift,emilstahl/swift,arvedviehweger/swift,calebd/swift,stephentyrone/swift,jopamer/swift,cbrentharris/swift,gribozavr/swift,bitjammer/swift,shajrawi/swift,LeoShimonaka/swift,SwiftAndroid/swift,khizkhiz/swift,IngmarStein/swift,swiftix/swift,roambotics/swift,tardieu/swift,CodaFi/swift,apple/swift,adrfer/swift,jopamer/swift,xedin/swift,swiftix/swift,zisko/swift,ahoppen/swift,bitjammer/swift,cbrentharris/swift,JaSpa/swift,allevato/swift,zisko/swift,SwiftAndroid/swift,codestergit/swift,danielmartin/swift,jckarter/swift,ben-ng/swift,gribozavr/swift,JGiola/swift,calebd/swift,austinzheng/swift,roambotics/swift,felix91gr/swift,huonw/swift,Ivacker/swift,frootloops/swift,codestergit/swift,therealbnut/swift,OscarSwanros/swift,rudkx/swift,aschwaighofer/swift,ken0nek/swift,johnno1962d/swift,parkera/swift,kentya6/swift,parkera/swift,sdulal/swift,hughbe/swift,apple/swift,brentdax/swift,kperryua/swift,swiftix/swift.old,return/swift,gregomni/swift,Jnosh/swift,KrishMunot/swift,gmilos/swift,gregomni/swift,MukeshKumarS/Swift,amraboelela/swift,aschwaighofer/swift,atrick/swift,arvedviehweger/swift,xwu/swift,atrick/swift,shajrawi/swift,lorentey/swift,amraboelela/swift,sdulal/swift,stephentyrone/swift,LeoShimonaka/swift,swiftix/swift,dreamsxin/swift,shajrawi/swift,deyton/swift,SwiftAndroid/swift,nathawes/swift,nathawes/swift,practicalswift/swift,djwbrown/swift,tjw/swift,frootloops/swift,kperryua/swift,uasys/swift,zisko/swift,shahmishal/swift,codestergit/swift,roambotics/swift,gregomni/swift,MukeshKumarS/Swift,CodaFi/swift,djwbrown/swift,bitjammer/swift,IngmarStein/swift,tinysun212/swift-windows,emilstahl/swift,sdulal/swift,natecook1000/swift,kusl/swift,jopamer/swift,karwa/swift,kperryua/swift,mightydeveloper/swift,kentya6/swift,kstaring/swift,Jnosh/swift,russbishop/swift,khizkhiz/swift,jmgc/swift,khizkhiz/swift,tardieu/swift,swiftix/swift.old,stephentyrone/swift,hooman/swift,sschiau/swift,devincoughlin/swift,calebd/swift,shahmishal/swift,modocache/swift,parkera/swift,djwbrown/swift,manavgabhawala/swift,return/swift,zisko/swift,allevato/swift,lorentey/swift,adrfer/swift,glessard/swift,dduan/swift,airspeedswift/swift,OscarSwanros/swift,arvedviehweger/swift,allevato/swift,huonw/swift,milseman/swift,karwa/swift,felix91gr/swift,SwiftAndroid/swift,gribozavr/swift,slavapestov/swift,devincoughlin/swift,nathawes/swift,shahmishal/swift,karwa/swift,therealbnut/swift,deyton/swift,kusl/swift,ben-ng/swift,shajrawi/swift,tinysun212/swift-windows,harlanhaskins/swift,karwa/swift,xedin/swift,ahoppen/swift,KrishMunot/swift,dduan/swift,Ivacker/swift,jmgc/swift,slavapestov/swift,jopamer/swift,johnno1962d/swift,johnno1962d/swift,SwiftAndroid/swift,xwu/swift,adrfer/swift,Ivacker/swift,lorentey/swift,xwu/swift,huonw/swift,ahoppen/swift,kusl/swift,austinzheng/swift,hooman/swift,kperryua/swift,stephentyrone/swift,frootloops/swift,kperryua/swift,harlanhaskins/swift,swiftix/swift.old,rudkx/swift,gottesmm/swift,natecook1000/swift,gottesmm/swift,hooman/swift,khizkhiz/swift,modocache/swift,ken0nek/swift,aschwaighofer/swift,KrishMunot/swift,tjw/swift,CodaFi/swift,LeoShimonaka/swift,SwiftAndroid/swift,hughbe/swift,sdulal/swift,practicalswift/swift,lorentey/swift,jopamer/swift,ahoppen/swift,djwbrown/swift,sschiau/swift,swiftix/swift.old,modocache/swift,uasys/swift,tjw/swift,jckarter/swift,harlanhaskins/swift,CodaFi/swift,aschwaighofer/swift,jtbandes/swift,kentya6/swift,cbrentharris/swift,brentdax/swift,therealbnut/swift,xwu/swift,emilstahl/swift,adrfer/swift,roambotics/swift,russbishop/swift,arvedviehweger/swift,codestergit/swift,ken0nek/swift,KrishMunot/swift,OscarSwanros/swift,kstaring/swift,adrfer/swift,hughbe/swift,OscarSwanros/swift,KrishMunot/swift,kentya6/swift,therealbnut/swift,MukeshKumarS/Swift,gmilos/swift,djwbrown/swift,karwa/swift,practicalswift/swift,tinysun212/swift-windows,gmilos/swift,felix91gr/swift,johnno1962d/swift,return/swift,KrishMunot/swift,gottesmm/swift,ken0nek/swift,dduan/swift,alblue/swift,dduan/swift,hughbe/swift,Ivacker/swift,benlangmuir/swift,gregomni/swift,jckarter/swift,aschwaighofer/swift,mightydeveloper/swift,bitjammer/swift,khizkhiz/swift,allevato/swift,aschwaighofer/swift,LeoShimonaka/swift,dduan/swift,khizkhiz/swift,uasys/swift,dreamsxin/swift,shahmishal/swift,ken0nek/swift,brentdax/swift,tjw/swift,gribozavr/swift,ahoppen/swift,danielmartin/swift,natecook1000/swift,stephentyrone/swift,airspeedswift/swift,manavgabhawala/swift,swiftix/swift.old,uasys/swift,natecook1000/swift,slavapestov/swift,austinzheng/swift,jopamer/swift,huonw/swift,devincoughlin/swift,sschiau/swift,brentdax/swift,jmgc/swift,devincoughlin/swift,kentya6/swift,swiftix/swift.old,dduan/swift,MukeshKumarS/Swift,IngmarStein/swift,khizkhiz/swift,JaSpa/swift,johnno1962d/swift