[LoopUnroll] Support loops with exiting block that is neither header nor

latch.

Summary: Remove the limitation in LoopUnrollPass that exiting block must
be either header or latch.
Reviewer: dmgreen, jdoerfert, Meinersbur, kbarton, bmahjour, etiotto,
fhahn, efriedma
Reviewed By: etiotto, fhahn, efriedma
Subscribers: efriedma, lkail, xbolva00, hiraditya, zzheng, llvm-commits
Tag: LLVM
Differential Revision: https://reviews.llvm.org/D80477