[SCEV] Strenthen nowrap flags after constant folding for mul exprs

[SCEV] Strenthen nowrap flags after constant folding for mul exprs

Same change as 0dda6333175c1749f12be660456ecedade3bcf21, but for
mul expressions. We want to first fold any constant operans and
then strengthen the nowrap flags, as we can compute more precise
flags at that point.