[SCEV] Move SCEVLostPoisonFlags() check into SCEVExpander

Always insert values into ExprValueMap, and instead skip using them
in SCEVExpander if poison-generating flags have been lost. This
ensures that all values that are in ValueExprMap are also in
ExprValueMap, so we can use the latter to invalidate the former.

This change is probably not entirely NFC for the case where
originally the SCEV had no nowrap flags but they were inferred
later, in which case that would now allow reusing the existing
value for expansion.

Differential Revision: https://reviews.llvm.org/D112389