[llvm] [LoopPeel] LCSSA form is destroyed by LoopPeel, preserve it (PR #78696)

[via llvm-commits]

annamthomas wrote:

Quick question - I had seen a bug previously (and had a fix in phabricator, which I cannot access now). The issue is that `canPeel` doesn't check for LCSSA form, where as in fact peeling requires loop to be in LCSSA form. Can you check in your case if the loop already was in LCSSA form before peeling? What I described might be an entirely different issue (I could only trigger it from llvm unit tests at the time).

https://github.com/llvm/llvm-project/pull/78696