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

[Matt Arsenault via llvm-commits]

arsenm wrote:

Test really high powers of 2. Particularly the cases where the exponent is > 64 (or < -16) and thus no longer an inline immediate 

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