[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:

Code change looks right (though it's missing the mirror change for GlobalISel in AMDGPUCallLowering). This will need test updates though 

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