RFR: 8215985: ZGC: Simplify reference processing in light of JDK-8175797

[Kim Barrett]

Please review this change to ZGC reference processing to take
advantage of the changes made by JDK-8175797, which made
Reference.enqueue clear the referent and made Reference.enqueue
unsupported by FinalReference.

As a result of JDK-8175797, the special treatment of the 'next' field
by the GC is no longer needed for most reference type; only
FinalReference needs special handling of that field.  This means
(except for FinalReference, e.g. REF_FINAL) the 'next' field no longer
needs to be examined by discovery, and does not need to be self-looped
by notification (eliminating a CAS to deal with possible concurrent
enqueue).

In discover_reference, the order of tests was changed because the
inactive final reference case should be relatively rare.  Doing the
"strongly alive or null" test first should be a better order, with the
remaining tests being for somewhat uncommon special cases that would
pass that discoverability test.

CR:
https://bugs.openjdk.java.net/browse/JDK-8215985

Webrev:
http://cr.openjdk.java.net/~kbarrett/8215985/open.00/

Testing:
mach5 tier{1-4}, hs-tier{5-8}-gc.