[AMDGPU] Use aperture registers instead of S_GETREG

Fixes a longstanding TODO in the codebase where we were using S_GETREG + shift to do something that could simply be done with an inline constant (register).

Patch based on D31874 by @kzhuravl
Depends on D137767

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D137542