[llvm] [AMDGPU] Add intrinsic exposing s_alloc_vgpr (PR #163951)

[Jannik Silvanus via llvm-commits]

https://github.com/jasilvanus commented:

The TableGen and test changes look good to me, but I'm not familiar with the backend C++ changes, so I'm leaving approval for someone else.

https://github.com/llvm/llvm-project/pull/163951