RFR: 8339960: GenShen: Fix inconsistencies in generational Shenandoah behavior

[Kelvin Nilsen]

This pull request contains a backport of commit 1a7d597d from the openjdk/shenandoah repository.

The commit being backported was authored by Kelvin Nilsen on 13 Sep 2024 and was reviewed by William Kemper.

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Commit messages:
 - Backport 1a7d597dad0fb3560f88f582ab760cca28cf54fd

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/90/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=90&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339960
  Stats: 28 lines in 1 file changed: 20 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/90.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/90/head:pull/90

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/90