Undiscovered references if using ReferentBasedDiscovery

The current implementation of `ReferentBasedDiscovery` would sometime incorrectly mark a referent strongly reachable, causing the enclosing non-strong ref undiscovered.

Reproducer: `make CONF=debug test TEST=gc/g1/humongousObjects/TestHumongousClassLoader.java JTREG="VM_OPTIONS=-XX:RefDiscoveryPolicy=1"`

The problematic path is hit only by G1. Serial and Parallel collectors are not affected.