[llvm] Update amdgpu_gfx functions to use s0-s3 for inreg SGPR arguments on targets using scratch instructions for stack (PR #78553)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> How should the tests be added, I see too many of them failing at the moment?

This should be covered by existing tests (which just now need many updates)

https://github.com/llvm/llvm-project/pull/78553