Integrated: 8338528: GenShen: Cleanup shenandoahHeapRegion

[William Kemper]

Clean backport.

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Commit messages:
 - Backport 27d0e617f97be03fae0b110ca58a3860c15d5679

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/88/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=88&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338528
  Stats: 81 lines in 6 files changed: 31 ins; 28 del; 22 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/88.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/88/head:pull/88

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/88