Add an assert to createPlusZeroBorrow
Add an assert to createPlusZeroBorrow

If this doesn't hold, it silently miscompiles.

roambotics/swift,gregomni/swift,glessard/swift,ahoppen/swift,atrick/swift,xwu/swift,ahoppen/swift,benlangmuir/swift,rudkx/swift,roambotics/swift,gregomni/swift,rudkx/swift,atrick/swift,apple/swift,xwu/swift,JGiola/swift,xwu/swift,rudkx/swift,ahoppen/swift,apple/swift,apple/swift,ahoppen/swift,gregomni/swift,benlangmuir/swift,atrick/swift,apple/swift,benlangmuir/swift,roambotics/swift,JGiola/swift,atrick/swift,roambotics/swift,xwu/swift,JGiola/swift,ahoppen/swift,apple/swift,rudkx/swift,JGiola/swift,gregomni/swift,ahoppen/swift,rudkx/swift,glessard/swift,JGiola/swift,roambotics/swift,JGiola/swift,roambotics/swift,benlangmuir/swift,gregomni/swift,apple/swift,gregomni/swift,atrick/swift,atrick/swift,benlangmuir/swift,glessard/swift,xwu/swift,glessard/swift,xwu/swift,xwu/swift,glessard/swift,rudkx/swift,glessard/swift,benlangmuir/swift