[llvm] [AMDGPU] Constant fold FMAD_FTZ (PR #69443)

[Jay Foad via llvm-commits]

https://github.com/jayfoad commented:

Make sense to me but please check with @arsenm.

For reference FMAD_FTZ was introduced by https://reviews.llvm.org/D29958.

https://github.com/llvm/llvm-project/pull/69443