RFR: 8331609: GenShen: Refactor generational mode allocations [v4]

[Kelvin Nilsen]

On Wed, 8 May 2024 23:23:28 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> * The code for resizing and retrying young allocation requests according to what's available in the young generation has been removed. This code became redundant after the free set was rewritten and began providing the same functionality.
>> * An unused field and its attendant methods were removed from `ShenandoahThreadLocalData`.
>> * The code for managing `plab` expenditures and configuring other plab behavior has been moved from `ShenandoahHeap` into `ShenandoahOldGeneration`.
>
> William Kemper has updated the pull request incrementally with three additional commits since the last revision:
> 
>  - Improve comments, remove too strong assert
>  - Initialize _evacuation_reserve member
>  - Modify test fixture to avoid accessing ShenandoahHeap before we know if UseShenandoahGC is set

Looks good.  Thanks.

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Marked as reviewed by kdnilsen (Committer).

PR Review: https://git.openjdk.org/shenandoah/pull/427#pullrequestreview-2048367748