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

[Vedant Paranjape via llvm-commits]

vedantparanjape-amd wrote:

> Can you please explain in what way LCSSA form gets broken? How does the IR look like after peeling but before simplification?
> 
> Just calling formLCSSARecursively() is of course correct, but it's better to understand why LCSSA gets broken and whether we can fix it incrementally instead.

![red](https://github.com/llvm/llvm-project/assets/126445630/5d560290-b00b-4c5c-8adf-aa271b42706c)

This is the CFG and below is the diff at the end of LoopPeel before and after LCSSA form is regenerated.

![image](https://github.com/llvm/llvm-project/assets/126445630/975a8706-abc4-4328-ad15-807d2eca095c)


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