[LoopInfo] Introduce getUniqueNonLatchExitBlocks utility function

Extract the code from LoopUnrollRuntime into utility function to
re-use it in D63923.

Reviewers: reames, mkuper
Reviewed By: reames
Subscribers: fhahn, hiraditya, zzheng, dmgreen, llvm-commits
Differential Revision: https://reviews.llvm.org/D64548

llvm-svn: 366040