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

[Shilei Tian via llvm-commits]

shiltian wrote:

> I'm not sure I trust this.

Which part? https://github.com/llvm/llvm-project/blob/83fe85115da9dc25fa270d2ea8140113c8d49670/llvm/lib/Target/AMDGPU/AMDGPUISelDAGToDAG.cpp#L3481

> It might be better to directly check the constant bus restriction, similar to how ThreeOpFrag does

I tried this as you suggested but because the pattern here is a `ComplexPattern` (or `GIComplexOperandMatcher`), it doesn't have support for checking predicate, `GISelPredicateCode` as what `ThreeOpFrag` is used.

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