[AMDGPU] Avoid CodeGen dependencies from AMDGPU/Utils and MCTargetDesc

This required two substantial changes:
1. Moving a `getRegBitWidth(TargetRegisterClass)` overload out of Utils
   and into CodeGen
2. Passing the string function name to AMDGPUPALMetadata instead of the
   MachineFunction

Other changes are minor or updates to accommodate the first two.

See issue #64166 for more information on the layering issue.

Differential Revision: https://reviews.llvm.org/D156486