[llvm] [InstCombine] Implement `fcmp (fadd x, 0.0), y` => `fcmp x, y` optimization (PR #88476)

[Vlad Mishel via llvm-commits]

vmishelcs wrote:

@dtcxzyw Please take a look whenever you have time :) Let me know if I should make any changes.

Question: Should I also add tests for the case of `fcmp (fsub x, 0.0), y`? From what I can tell my implementation already covers this case. Is this because somewhere in the code we transform `fsub x, 0.0` => `fadd x, 0.0`?

https://github.com/llvm/llvm-project/pull/88476