[llvm] [AMDGPU] Add VGPR MSB-affinity pass to reduce s_set_vgpr_msb on gfx1250 (PR #216909)

[Shilei Tian via llvm-commits]

shiltian wrote:

I'm surprised that this new pass doesn't cause any actual test changes. We will definitely need **some** to justify this pass, preferably pre-commit them first and then show the diff in this PR.

https://github.com/llvm/llvm-project/pull/216909