[llvm] [CodeGen] [AMDGPU] Adds pre-commit test for fmul-select combine (PR #111107)

[Matt Arsenault via llvm-commits]

arsenm wrote:

You can always write something simpler and let constant folding show you how it should be printed: https://godbolt.org/z/PsdMnq37T

https://github.com/llvm/llvm-project/pull/111107