[DAGCombiner] Don't skip no overflow check on UMULO if the first...

[DAGCombiner] Don't skip no overflow check on UMULO if the first computeKnownBits call doesn't return any 0 bits.

Even if the first computeKnownBits call doesn't have any zero
bits it is possible the other operand has bitwidth-1 leading zero.
In that case overflow is still impossible. So always call computeKnownBits
for both operands.