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

[Lucas Ramirez via llvm-commits]

lucas-rami wrote:

Thanks @shiltian and @rampitec for the feedback, I understand the problem.

I tried to reduce the contribution's scope to only defer the creation of (`S_NOP`/)`S_SET_VGPR_MSB` after a `S_SETREG_IMM32_B32` but it doesn't integrate very cleanly on its own so I'll include it as part of some larger future patch that will optimize `S_SET_VGPR_MSB` insertion more broadly. 

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