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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> 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.

Anyone, including yourself, can dismiss those 

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