[SCEV] Track backedge taken count users (NFCI)

Track which SCEVs are used as ExactNotTaken counts in
BackedgeTakenInfo structures, so we can directly determine which
loops need to be invalidated, rather than iterating over all BECounts.

This gives a small compile-time improvement on average, but the
motivation here is more to ensure there are no degenerate cases,
if the number of backedge taken counts is large.

Differential Revision: https://reviews.llvm.org/D114784