RFR(S): type system fix

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/valhalla/lworld-typesystem-fix/webrev.00/

This allows a test that I added for copyOf to run correctly. The change
attempts to mirror the logic for instances. I tried to run our tests
without the workaround in Type::meet_helper() and I hit no failure.

Roland.