RFR: Borrow from old gen [v3]

[William Kemper]

On Tue, 8 Feb 2022 17:26:30 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> This commit represents multiple months of incremental performance improvements to allow generational shenandoah to run more efficiently, especially with larger heap sizes and high memory utilization.
>> 
>> Specific improvements are described in individual commit log messages.
>
> Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fixup mistakes from previous commit

Marked as reviewed by wkemper (Committer).

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PR: https://git.openjdk.java.net/shenandoah/pull/110