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

[Petar Avramovic via llvm-commits]

petar-avramovic wrote:

> > IMO, the easiest approach is to force vgpr reg class on all operands when selecting vop3p instructions.
> 
> IIUC, this PR made a right move by removing the copy stripping.

What I meant was to constrainOperandRegClass to vgpr after adding sgpr operand to MIB. This way there is no need to avoid copy stripping and final asm code stays the same. Didn't try to implement it, so it may not be that easy.


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