[SCEV] Verify all IR -> SCEV mappings

This extends SCEV verification to check not only backedge-taken
counts, but all entries in the IR -> SCEV cache. The restrictions
are the same as for the BECount case, i.e. we ignore expressions
based on undef, we only diagnose constant deltas (there are way
too many false positives otherwise) and we limit to reachable code.

Differential Revision: https://reviews.llvm.org/D121104