Integrated: 8331609: GenShen: Refactor generational mode allocations

[William Kemper]

Clean backport.

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Commit messages:
 - Backport 80e69877f807499e811ff0353d7477057fa2f63d

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/46/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=46&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331609
  Stats: 503 lines in 8 files changed: 272 ins; 175 del; 56 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/46.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/46/head:pull/46

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/46