[ShrinkBorrowScope] Replaced check with assertion.
[ShrinkBorrowScope] Replaced check with assertion.

Previously, it was checked whether a barrier terminator was actually a
control flow terminator before adding end_borrows at the beginnings of
its successors.  But it can't actually happen that a terminator is
classified as a barrier without the beginnings of all of its
predecessors having been reached because the BackwardReachability data
flow is pessimistic and only visits the end of a block (i.e. its
terminator) if it reached the beginnings of all of that block's
successors (see BackwardReachability::meetOverSuccessors and where the
data flow calls it).

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