[llvm] [WIP][AMDGPU] Change `CC_AMDGPU_Func` to only use SGPR0 to SGPR27 for `inreg` argument passing (PR #115753)

[Shilei Tian via llvm-commits]

shiltian wrote:

This PR is still WIP as it hits some other invalid SGPR to VGPR copy, but I'd like to collect some early feedback before I spend much effort on it.

https://github.com/llvm/llvm-project/pull/115753