[LoopUnrollRuntime] NFC: use the precomputed loop exit in ConnectProlog

[LoopUnrollRuntime] NFC: use the precomputed loop exit in ConnectProlog

Minor refactoring to use the preexisting loop exit that's already
calculated. We do not need to recompute the loop exit in ConnectProlog.
Apart from avoiding redundant computation, this is required for
supporting multiple loop exits when Prolog remainder loops are generated.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307417 91177308-0d34-0410-b5e6-96231b3b80d8