Fix #548: use identityHashCode for better performance
Fix #548: use identityHashCode for better performance

The two related Java issues are marked as resolved:

- https://bugs.openjdk.java.net/browse/JDK-8068239
- https://bugs.openjdk.java.net/browse/JDK-8130042

There are no actual fix, but it's mentioned not reproducible.

som-snytt/dotty,dotty-staging/dotty,som-snytt/dotty,lampepfl/dotty,dotty-staging/dotty,som-snytt/dotty,dotty-staging/dotty,dotty-staging/dotty,lampepfl/dotty,som-snytt/dotty,lampepfl/dotty,lampepfl/dotty,sjrd/dotty,lampepfl/dotty,som-snytt/dotty,sjrd/dotty,sjrd/dotty,dotty-staging/dotty,sjrd/dotty,sjrd/dotty