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
For each of the x.with.overflow variants, if only the overflow bit is consumed, we can generate a direct overflow comparison. This precommits tests for each of the variants and tries to cover interesting cornercases.
