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

[Shilei Tian via llvm-commits]

shiltian 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.

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