[M68k] Optimize for overflow arithmetics that will never overflow

We lower overflow arithmetics to its M68kISD counterparts that produce
results of {i16/i32, i8} in which the second resut represents CCR. In
the event where we're certain there won't be an overflow, for instance
8 & 16-bit multiplications, we simply use zero in replacement of the
second result.
This patch replaces M68kISD::CMOV that takes this kind of zero or
all-ones CCR as condition value with its corresponding operand value.