RFR: 8334445: Parallel: Decouple maximum compaction from SoftReference clearing

[Albert Mingkun Yang]

Simple refactoring to not assume maximum-compaction from soft-ref-clearing.

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Commit messages:
 - pgc-max-compact

Changes: https://git.openjdk.org/jdk/pull/19763/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19763&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334445
  Stats: 45 lines in 4 files changed: 7 ins; 14 del; 24 mod
  Patch: https://git.openjdk.org/jdk/pull/19763.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19763/head:pull/19763

PR: https://git.openjdk.org/jdk/pull/19763