[AMDGPU] Add symbolic names for gfx940 HWREGs

The namespaces of HWREGs is now overlapping with gfx10. Thus the
patch is longer than necessary to just support new names. It also
need to handle proper error messages, i.e. to issue a "specified
hardware register is not supported on this GPU" message.

This may need a major refactoring in the future.

Differential Revision: https://reviews.llvm.org/D121418