[llvm] [AMDGPU][Attributor] Infer inreg attribute in `AMDGPUAttributor` (PR #146720)

[Shilei Tian via llvm-commits]

shiltian wrote:

This is a trivial version of https://github.com/llvm/llvm-project/pull/101609, as requested by @arsenm.

https://github.com/llvm/llvm-project/pull/146720