[SCEV] Track and invalidate ValuesAtScopes users

ValuesAtScopes maps a SCEV and a Loop to another SCEV. While we
invalidate entries if the left-hand SCEV is invalidated, we
currently don't do this for the right-hand SCEV. Fix this by
tracking users in a reverse map and using it for invalidation.

This is conceptually the same change as D114738, but using the
reverse map to avoid performance issues.

Differential Revision: https://reviews.llvm.org/D114788