Replace a condition with an assert.
Replace a condition with an assert.

This makes it easier to continue to simplify this code, and it should be
reasonable to maintain this invariant in the future.

Swift SVN r27030

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