[llvm] Update amdgpu_gfx functions to use s0-s3 for inreg SGPR arguments on targets using scratch instructions for stack #78226 (PR #81394)

[via llvm-commits]

SahilPatidar wrote:

> > @arsenm, why are some test updates crashing with this output: Assertion failed: (Reg != AMDGPU::NoRegister), function allocateFixedSGPRInputImpl, file /Users/sahilpatidar/Desktop/llvm/llvm-project/llvm/lib/Target/AMDGPU/SIISelLowering.cpp, line 2292.
> 
> That indicates a free register wasn't found available for an input. What do the function signatures look like for the cases that hit this?

for this test function `define void @void_func_a13i32_inreg([13  x i32] inreg %arg0, ptr addrspace(1) %ptr)`

https://github.com/llvm/llvm-project/pull/81394