RFR: 8314329: AgeTable: add is_clear() & allocation spec, and relax assert to allow use of 0-index slot [v3]

[Aleksey Shipilev]

On Fri, 19 Jan 2024 23:32:42 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

>> I'd like to propose the following changes to the AgeTable (motivated by the uses in generational shenandoah):
>
> Y. Srinivas Ramakrishna has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Changes from @albertnetymk review : used #ifndef PRODUCT rather than
>   PRODUCT_RETURN0 for is_clear().

Marked as reviewed by shade (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/17470#pullrequestreview-1836090357