Integrated: 8332094: GenShen: Reuse existing code to verify usage before rebuilding free set

[William Kemper]

Clean backport

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Commit messages:
 - Backport 78836a579d83a98eab2e1431a1e7a20ea0d79149

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/47/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=47&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332094
  Stats: 104 lines in 5 files changed: 11 ins; 62 del; 31 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/47.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/47/head:pull/47

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/47