[llvm] [AMDGPU][Attributor] Infer `inreg` attribute in `AMDGPUAttributor` (PR #101609)

[Matt Arsenault via llvm-commits]

arsenm wrote:

inreg argument generation is busted, and this cannot land until that is fixed. e.g. https://godbolt.org/z/joeTjKbf7 is clobbering the SRD 

https://github.com/llvm/llvm-project/pull/101609