[llvm] [AMDGPU][CodeGen] Don't always create `S_SET_VGPR_MSB` after `S_SETREG_B32` (PR #210042)

[Shilei Tian via llvm-commits]

shiltian wrote:

> Furthermore, if the VGPR MSBs induced by the MODE register mode are appropriate for further instructions, we simply use them without inserting a new `S_SET_VGPR_MSB`.

IIRC, that's what I did before, but later we explicitly decided not to want that, because we don't want to rely on this for correct execution behavior.

https://github.com/llvm/llvm-project/pull/210042