[LoopDeletion] Add an assert that verifies LCSSA

This is inspired by PR24804 -- had this assert been there before,
isolating the root cause for PR24804 would have been far easier.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@261481 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed