AMDGPU: Correct operand types for v_mad_mix*

These aren't really packed instructions, so the default
op_sel_hi should be 0 since this indicates a conversion.
The operand types are scalar values that behave similar
to an f16 scalar that may be converted to f32.

Doesn't change the default printing for op_sel_hi, just
the parsing.

llvm-svn: 312179