[M68k][Disassembler] Fix decoding conflict

This diff fixes decoding conflict between these pair of instructions:

ADD(16|32)dd / ADD(16|32)dr
SUB(16|32)dd / SUB(16|32)dr
AND(16|32)dd / AND(16|32)dr
OR(16|32)dd  / OR(16|32)dr

Reviewed By: ricky26

Differential Revision: https://reviews.llvm.org/D125861