JDK-8175797 changed Reference.enqueue to ensure the reference object is cleared before enqueuing it. This eliminates some states and state transitions, allowing some simplification of reference processing.

In particular, for reference processing when discovery is not atomic, the case of a discovered reference which is later found to have been enqueued but still has a non-null referent is no longer possible.  This means that much of the difference between pp2_work and pp2_work_concurrent_discovery is no longer required, and those functions can be mostly merged.

For the most part, the Reference.next field no longer needs any special treatment, and can be handled like any normal object field.  It still requires some special handling in support of FinalReferences though; a non-null next is the mechanism used to indicate a FinalReference is no longer active, since the reference object is not cleared.