[SCEV] Stop applying contextual flags in applyLoopGuards

This fixes a violation of the wrap flag rules introduced in c4048d8f. As noted in the original review, the NUW is legal to infer from the structure of the replacee, but a) there's no test coverage, and b) this should be done generically for all multiplies.

Differential Revision: https://reviews.llvm.org/D109782