[llvm] ValueTracking: Check if x is undef for fma(x, x, y) analysis (PR #174763)

[Matt Arsenault via llvm-commits]

arsenm wrote:

https://github.com/llvm/llvm-project/pull/174458 and https://github.com/llvm/llvm-project/pull/174453 cover the related cases 

https://github.com/llvm/llvm-project/pull/174763