Fix a latent bug in RCIdentityAnalysis.getUsers.
Fix a latent bug in RCIdentityAnalysis.getUsers.

This bug was introduced by the migration to multi-value SILInstructions.

This fix will be tested by copy forwarding.

WARNING: To handle the potential existence of multi-value
instructions, don't simply iterate over the results. You may be
inadvertently skipping instructions with no results.

Details:

RCIdentityFunctionInfo::getRCUsers searches the def-use chain to find
all users. When visiting a use, it checks the result to determine if
it's casting or extracting the original reference operand. If, so it
continues along the def-use chain. So, it simply wasn't visiting
instructions with no results.

I inverted the logic so that the special case now requires identifying
a SingleValueInstruction, while the default case is conservative.

devincoughlin/swift,stephentyrone/swift,danielmartin/swift,parkera/swift,roambotics/swift,hooman/swift,harlanhaskins/swift,tkremenek/swift,tkremenek/swift,huonw/swift,practicalswift/swift,parkera/swift,tjw/swift,apple/swift,practicalswift/swift,gregomni/swift,CodaFi/swift,shajrawi/swift,benlangmuir/swift,harlanhaskins/swift,karwa/swift,jmgc/swift,sschiau/swift,sschiau/swift,apple/swift,shahmishal/swift,gregomni/swift,xedin/swift,aschwaighofer/swift,brentdax/swift,gribozavr/swift,danielmartin/swift,airspeedswift/swift,rudkx/swift,jmgc/swift,practicalswift/swift,lorentey/swift,alblue/swift,gregomni/swift,hooman/swift,ahoppen/swift,allevato/swift,karwa/swift,stephentyrone/swift,JGiola/swift,benlangmuir/swift,stephentyrone/swift,jopamer/swift,shajrawi/swift,natecook1000/swift,danielmartin/swift,hooman/swift,CodaFi/swift,aschwaighofer/swift,huonw/swift,airspeedswift/swift,jckarter/swift,jckarter/swift,glessard/swift,xwu/swift,devincoughlin/swift,jopamer/swift,atrick/swift,sschiau/swift,parkera/swift,amraboelela/swift,practicalswift/swift,alblue/swift,gribozavr/swift,tkremenek/swift,huonw/swift,xedin/swift,jmgc/swift,atrick/swift,alblue/swift,huonw/swift,benlangmuir/swift,jckarter/swift,karwa/swift,danielmartin/swift,lorentey/swift,shajrawi/swift,xwu/swift,nathawes/swift,practicalswift/swift,gribozavr/swift,devincoughlin/swift,airspeedswift/swift,ahoppen/swift,tjw/swift,lorentey/swift,karwa/swift,shahmishal/swift,brentdax/swift,shahmishal/swift,glessard/swift,practicalswift/swift,harlanhaskins/swift,danielmartin/swift,parkera/swift,natecook1000/swift,xedin/swift,parkera/swift,alblue/swift,tkremenek/swift,xedin/swift,alblue/swift,tjw/swift,lorentey/swift,hooman/swift,austinzheng/swift,xedin/swift,sschiau/swift,allevato/swift,allevato/swift,atrick/swift,glessard/swift,stephentyrone/swift,parkera/swift,JGiola/swift,tjw/swift,shahmishal/swift,airspeedswift/swift,austinzheng/swift,shahmishal/swift,allevato/swift,stephentyrone/swift,tkremenek/swift,xwu/swift,allevato/swift,rudkx/swift,roambotics/swift,hooman/swift,aschwaighofer/swift,austinzheng/swift,rudkx/swift,brentdax/swift,shajrawi/swift,devincoughlin/swift,airspeedswift/swift,stephentyrone/swift,jckarter/swift,gribozavr/swift,shajrawi/swift,harlanhaskins/swift,CodaFi/swift,JGiola/swift,natecook1000/swift,karwa/swift,practicalswift/swift,roambotics/swift,roambotics/swift,atrick/swift,devincoughlin/swift,JGiola/swift,alblue/swift,jmgc/swift,sschiau/swift,gribozavr/swift,gribozavr/swift,atrick/swift,austinzheng/swift,shahmishal/swift,nathawes/swift,brentdax/swift,austinzheng/swift,rudkx/swift,ahoppen/swift,apple/swift,ahoppen/swift,roambotics/swift,harlanhaskins/swift,natecook1000/swift,aschwaighofer/swift,jopamer/swift,amraboelela/swift,stephentyrone/swift,ahoppen/swift,brentdax/swift,austinzheng/swift,CodaFi/swift,roambotics/swift,jckarter/swift,shajrawi/swift,glessard/swift,tkremenek/swift,hooman/swift,hooman/swift,JGiola/swift,CodaFi/swift,apple/swift,jmgc/swift,amraboelela/swift,nathawes/swift,jopamer/swift,natecook1000/swift,natecook1000/swift,huonw/swift,aschwaighofer/swift,shajrawi/swift,tkremenek/swift,CodaFi/swift,karwa/swift,shahmishal/swift,amraboelela/swift,apple/swift,huonw/swift,harlanhaskins/swift,xedin/swift,jckarter/swift,gregomni/swift,danielmartin/swift,nathawes/swift,rudkx/swift,harlanhaskins/swift,alblue/swift,xwu/swift,nathawes/swift,JGiola/swift,CodaFi/swift,aschwaighofer/swift,benlangmuir/swift,tjw/swift,airspeedswift/swift,austinzheng/swift,sschiau/swift,brentdax/swift,jopamer/swift,jmgc/swift,practicalswift/swift,gribozavr/swift,parkera/swift,sschiau/swift,benlangmuir/swift,shahmishal/swift,lorentey/swift,benlangmuir/swift,lorentey/swift,sschiau/swift,natecook1000/swift,atrick/swift,jopamer/swift,shajrawi/swift,tjw/swift,karwa/swift,xwu/swift,devincoughlin/swift,glessard/swift,xwu/swift,gribozavr/swift,ahoppen/swift,lorentey/swift,gregomni/swift,amraboelela/swift,allevato/swift,jckarter/swift,amraboelela/swift,xedin/swift,parkera/swift,aschwaighofer/swift,glessard/swift,karwa/swift,devincoughlin/swift,danielmartin/swift,jmgc/swift,devincoughlin/swift,lorentey/swift,amraboelela/swift,xwu/swift,apple/swift,xedin/swift,allevato/swift,gregomni/swift,nathawes/swift,tjw/swift,brentdax/swift,huonw/swift,rudkx/swift,jopamer/swift,nathawes/swift,airspeedswift/swift