RFR: 8329790: GenShen: Old regions that are pinned during final mark are not made parseable

[William Kemper]

Clean backport.

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Commit messages:
 - Backport 64b44a8c0e3a296d23905ac9e2233fc237057bab

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/38/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=38&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329790
  Stats: 40 lines in 3 files changed: 17 ins; 8 del; 15 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/38.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/38/head:pull/38

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/38