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

[Jay Foad via llvm-commits]

jayfoad wrote:

First, I don't understand the requirement, Are you trying to generate code that would run correctly on chips both with and without the bug? Why is that needed?

Second, if we _are_ trying to do that, then can't you simplify the setreg handling a lot more? We don't need any of the case 1, case 2, case 3 stuff, we can just forget any known MSBs every time we see a setreg.

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