[clang] [Clang][AMDGPU] Add a Sema check for the imm argument of ` __builtin_amdgcn_s_setreg` (PR #176838)

[Matt Arsenault via cfe-commits]

https://github.com/arsenm approved this pull request.

Probably should have made this i16imm to begin with 

https://github.com/llvm/llvm-project/pull/176838