RFR: 8336640: Shenandoah: Parallel worker use in parallel_heap_region_iterate

[William Kemper]

Clean backport.

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Commit messages:
 - Backport e74edbaea9f09169f597a470f647f3b7d10cc71b

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/108/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=108&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336640
  Stats: 21 lines in 2 files changed: 14 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/108.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/108/head:pull/108

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/108