[llvm] [AMDGPU] Fix VGPR to SGPR copy for inline asm with SGPR constraint (PR #176330)

[Matt Arsenault via llvm-commits]

arsenm wrote:

We shouldn't really support the r asm constraint. Or more realistically, we should treat it as an alias for v. 

https://github.com/llvm/llvm-project/pull/176330