RFR: 8327388: GenShen: census during marking is partial

[William Kemper]

Clean backport

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Commit messages:
 - Backport 10febd95d093165ca2e91c12e385e3754dc47ea0

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/57/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=57&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327388
  Stats: 166 lines in 11 files changed: 121 ins; 27 del; 18 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/57.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/57/head:pull/57

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/57