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

[Roman Kennke]

On Thu, 12 Sep 2024 20:23:36 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

> This fixes some bugs found in recent code review and playback of an assertion failure.
> 
> See also https://github.com/openjdk/shenandoah/pull/497

Looks good, thanks!

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Marked as reviewed by rkennke (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/20974#pullrequestreview-2307337108