[llvm] [AMDGPU] Fix setreg handling in the VGPR MSB lowering (PR #186491)

[Stanislav Mekhanoshin via llvm-commits]

rampitec wrote:

> Now I understand your motivation. You don't want to rely on the HW bug to get the indexing right, so you keep a list of `S_SETREG_IMM32_B32` between the current instruction and the last `S_SET_VGPR_MSB`. Each time, you try to piggyback on `S_SET_VGPR_MSB` instead of `S_SETREG_IMM32_B32`, and if it succeeds, you update all the `S_SETREG_IMM32_B32` in between.
> 
> This way, even if the HW issue is fixed in the future and we’'e not aware of it, this approach will still work. If we can make this behave as expected, I'm okay with this change.

Exactly.

https://github.com/llvm/llvm-project/pull/186491