[instcombine] Fold overflow check using umulo to comparison

If we have a umul.with.overflow where the multiply result is not used and one of the operands is a constant, we can perform the overflow check cheaper with a comparison then by performing the multiply and extracting the overflow flag.

(Noticed when looking at the conditions SCEV emits for overflow checks.)

Differential Revision: https://reviews.llvm.org/D104665