RFR: 8262973: Verify ParCompactionManager instance in PCAdjustPointerClosure

[Albert Mingkun Yang]

Verify the correct ParCompactionManager instance is used by VM/GC threads and some general cleanup.

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Commit messages:
 - verify

Changes: https://git.openjdk.java.net/jdk/pull/2813/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2813&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8262973
  Stats: 66 lines in 5 files changed: 37 ins; 19 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2813.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2813/head:pull/2813

PR: https://git.openjdk.java.net/jdk/pull/2813