[llvm] [AMDGPU][GlobalISel] Align `selectVOP3PMadMixModsImpl` with the `SelectionDAG` counterpart (PR #110168)

[Shilei Tian via llvm-commits]

shiltian wrote:

> Should have end to end IR test

How to make a IR test using `mad`? LLVM has `fma` but no `mad`. This sequence of code was reduced from the MIR sequence generated in #109295 but that PR is blocked by this one.

https://github.com/llvm/llvm-project/pull/110168