[AMDGPU] Fix predicates for V_DOT instructions. (#78198)

Resolves AsmParser ambiguities, e.g., between V_DOT4C_I32_I8_dpp_vi and
V_DOT4C_I32_I8_dpp_gfx10. The latter is predicated with isGFX10Only
while the first has no subtarget generation predicates.

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