RFR: 8274286: Skip null for make_referent_alive in referenceProcessor [v3]

[Albert Mingkun Yang]

> Simple change of avoiding a virtual call for null referent. With this change, two methods of processing the discovered list, `process_discovered_list_work` and `preclean_discovered_reflist`, share the same structure.
> 
> Test: hotspot_gc

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  comment

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5682/files
  - new: https://git.openjdk.java.net/jdk/pull/5682/files/e8d87db0..88d895d0

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5682&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5682&range=01-02

  Stats: 9 lines in 1 file changed: 7 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5682.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5682/head:pull/5682

PR: https://git.openjdk.java.net/jdk/pull/5682