[AMDGPU] Give some of the VI V_PK_* instructions a subtarget predicate. (#70334)

This resolves AsmParser ambiguity between them and similar instructions
for other subtargets, e.g., V_PK_SUB_U16_vi being identical to
V_PK_SUB_U16_gfx11 on GFX11.

Part of <https://github.com/llvm/llvm-project/issues/69256>.