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

[Stanislav Mekhanoshin via llvm-commits]

rampitec wrote:

> > OK, if the goal is to generate code that will work correctly with or without the hw bug then I agree this codegen makes sense.
> 
> You are still blocking the change.

And that block was based on my initial misunderstanding of where the logic went off for mask calculation, but that was addressed 2 months ago. The block is still there.

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