RFR: 8272905: Consolidate discovered lists processing

[Albert Mingkun Yang]

Simple change of merging discovered list processing for four kinds of non-strong refs and some general cleanup around.

Test: tier1-3

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Commit messages:
 - complete-gc

Changes: https://git.openjdk.java.net/jdk/pull/5242/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5242&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272905
  Stats: 125 lines in 3 files changed: 42 ins; 51 del; 32 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5242.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5242/head:pull/5242

PR: https://git.openjdk.java.net/jdk/pull/5242