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

[Vedant Paranjape via llvm-commits]

vedantparanjape-amd wrote:

@nikic I implemented a fix based on our discussion, I implemented the 2nd one, because it makes sure optimization doesn't bail out. Can you review it once, thanks !

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