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

[Nikita Popov via llvm-commits]

nikic wrote:

> Now we are unaware if such a transformation is expected of simplifyLoopIV and if it is also expected to maintain LCSSA.

simplifyLoopIV is supposed to maintain LCSSA form.

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