RFR: 8318057: GenShen: investigate small refactor of remset swap code

[Kelvin Nilsen]

On Tue, 17 Oct 2023 23:26:31 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

> Some closures were being unnecessarily applied to free regions rather than just to allocated old and young regions. Restrict the closure application to old and young regions as appropriate, eliminating extra generation affiliation checks in some closures. Also fixed a slightly incorrect documentation comment.
> 
> **Testing:**
> - jtreg and stress tests in progress in test pipeline helper
> - locally tested with jtreg hotsot_gc tests and with specjbb with and without heap verification
> - performance differences, if any, to be measured

Thanks for quick turnaround

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Marked as reviewed by kdnilsen (Committer).

PR Review: https://git.openjdk.org/shenandoah/pull/342#pullrequestreview-1686377737