[LoopUnroll] Don't peel loops where the latch isn't the exiting block

Peeling assumed this doesn't happen, but didn't check it.
This fixes PR32178.

Differential Revision: https://reviews.llvm.org/D30757


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@297993 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed