[AMDGPU] Create isGFX9Plus utility function

Introduce a utility function to make it more
convenient to write code that is the same on
the GFX9 and GFX10 subtargets.

Use isGFX9Plus in the AsmParser for AMDGPU.

Authored By: Joe_Nash

Differential Revision: https://reviews.llvm.org/D88908