RFR: 8332095: GenShen: Move more generational mode members out of shHeap

[William Kemper]

No conflicts after rebasing on other recent PRs

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Commit messages:
 - Backport d77f426b0a7c8b2c7285ee43d978be2cf584a51f

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/50/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=50&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332095
  Stats: 101 lines in 11 files changed: 39 ins; 44 del; 18 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/50.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/50/head:pull/50

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/50