RFR: 8332548: GenShen: Factor generational mode out of gc helpers [v2]

[Y. Srinivas Ramakrishna]

On Tue, 21 May 2024 22:10:41 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> Some of the generational mode support in `shenandoahConcurrentGC.cpp` and `shenandoahDegeneratedGC.cpp` can be factored into generational mode specific files and coalesced under fewer generational mode checks.
>
> William Kemper has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Templatize evact update task and closure to prevent excessive mode checks
>  - Use descriptive style in comments for consistency

LGTM; thanks for the changes!

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Marked as reviewed by ysr (Committer).

PR Review: https://git.openjdk.org/shenandoah/pull/436#pullrequestreview-2069720912