RFR: 8333109: GenShen: Factor generational mode out of gc helpers (redo) [v2]

[Kelvin Nilsen]

On Tue, 28 May 2024 19:14:57 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> This is the same as https://git.openjdk.org/shenandoah/pull/436, except that does _not_ remove a critical cancellation check. The comments around this cancellation check have been improved.
>
> William Kemper has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains two new commits since the last revision:
> 
>  - Restore cancellation check
>  - Restore orginal isolate-gc-helpers commit
>    
>    This reverts commit 894aefc077d495b3cd892d339e9ae9120cf3be54.

Marked as reviewed by kdnilsen (Committer).

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PR Review: https://git.openjdk.org/shenandoah/pull/441#pullrequestreview-2083878449