RFR: 8370667: GenShen: Only make assertions about region pinning for collected generation

[Y. Srinivas Ramakrishna]

On Tue, 28 Oct 2025 20:52:01 GMT, William Kemper <wkemper at openjdk.org> wrote:

> This broke recently because we expanded the scope of the assertion to check all regions. However, there are times when the pin status for a region is only updated if the region belongs to the generation being collected. This change restores the previous behavior of the assertion.

Marked as reviewed by ysr (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/28027#pullrequestreview-3401594703