[M68k] U/SMULd32d16 are not supposed to be communitive

M68k only has 16-bit x 16-bit -> 32-bit variant for multiplications
taking 16-bit operands. We still define two input operands for this
class of instructions, and tie the first operand to the result value.
The problem is that the two operands have different register classes
(DR32 and DR16) hence making these instructions communitive produces
invalid MachineInstr (though the final assembly will still be correct).