This adds validation for consistency of ValueExprMap and ExprValueMap, and fixes identified issues:
- Addrec construction directly wrote to ValueExprMap in a few places, without updating ExprValueMap. Add a helper to ensures they stay consistent. The adjustment in forgetSymbolicName() explicitly drops the old value from the map, so that we don't rely on it being overwritten.
- forgetMemoizedResultsImpl() was dropping the SCEV from ExprValueMap, but not dropping the corresponding entries from ValueExprMap.
This test changes because more instructions get reused by SCEV expander.