[SCEV] Verify that all mapped SCEV AddRecs refer to valid loops.

This check helps to guard against cases where expressions referring to
invalidated/deleted loops are not properly invalidated.

The additional check is motivated by the reproducer shared for 8fdac7cb
and I think in general make sense as a sanity check.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D88166