[llvm] [WIP][AMDGPU] Improve the handling of `inreg` arguments (PR #133614)

[Shilei Tian via llvm-commits]

shiltian wrote:

> Vaguely related, but we should also support inreg on function returns so we can avoid using the ugly hack of integer typed return values with shader calling conventions. Still not as nice as a proper per-field entry on the struct

That'd be in the follow up PRs.

https://github.com/llvm/llvm-project/pull/133614