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

[Rajveer Singh Bharadwaj via llvm-commits]

Rajveer100 wrote:

@arsenm 
It looks like @SahilPatidar is much [closer](https://github.com/llvm/llvm-project/pull/81394) to completion, closing this PR.

Will keep an eye on the same, to learn more!

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