[llvm] AMDGPU/GlobalISel: Handle inreg arguments as SGPRs (PR #78123)

[Jun Wang via llvm-commits]

https://github.com/jwanggit86 commented:

(1) Are sgpr0-sgpr3 reserved for some GPUs?
(2) A testcase where input args exhaust available SGPRs.

https://github.com/llvm/llvm-project/pull/78123