RFR: 8320331: G1 Full GC Heap verification relies on metadata not reset before verification

[Thomas Schatzl]

Hi all,

  please review this fix to full gc to properly set `parsable_bottom` for the heap regions marked through so that "during" verification (`-XX:+VerifyDuringGC`) does not access metadata already unlinked but not yet completely purged.

The test case reproduces crashes fairly well without the patch; the new test case is run with debug VMs only purely to save testing time.

Testing: gha, test case

Thanks,
  Thomas

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Commit messages:
 - Only run test case on debug VMs, sufficient
 - 8320331 g1 full gc "during" verification accesses half-unloaded metadata

Changes: https://git.openjdk.org/jdk/pull/16733/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16733&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320331
  Stats: 19 lines in 5 files changed: 16 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/16733.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16733/head:pull/16733

PR: https://git.openjdk.org/jdk/pull/16733