[SCEV] Fix ValuesAtScopesUsers consistency

Fixes verification failure reported at:
https://reviews.llvm.org/rGc9f9be0381d1

The issue is that getSCEVAtScope() might compute a result without
inserting it in the ValuesAtScopes map in degenerate cases,
specifically if the ValuesAtScopes entry is invalidated during the
calculation. Arguably we should still insert the result if no
existing placeholder is found, but for now just tweak the logic
to only update ValuesAtScopesUsers if ValuesAtScopes is updated.