RFR: 8333750: GenShen: Only instantiate young/old generations in generational mode

[William Kemper]

Clean backport

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Commit messages:
 - Backport 2abf4cb43e84dda883a38abc6bcbac5aba3d7ce8

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/59/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=59&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8333750
  Stats: 890 lines in 22 files changed: 471 ins; 323 del; 96 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/59.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/59/head:pull/59

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/59