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

[William Kemper]

On Wed, 11 Sep 2024 16:25:43 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

> This fixes some bugs found in recent code review and playback of an assertion failure.

Marked as reviewed by wkemper (Committer).

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PR Review: https://git.openjdk.org/shenandoah/pull/497#pullrequestreview-2304274268