[sil] Improve comment on swift::replaceSingleUse.
[sil] Improve comment on swift::replaceSingleUse.

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