RFR: Adaptive heuristic should take reserved memory into account [v2]

[William Kemper]

On Tue, 28 Jun 2022 00:03:24 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> Before this change, the heuristic considered the memory available to both the collector and mutators. Since the collector memory is not available to mutators, they may experience an allocation failure before the heuristic could anticipate. With this change, the heuristic looks at the memory available to mutators only. This has the effect of making the heuristic tend to start cycles earlier, thus avoiding degenerated cycles.
>
> William Kemper has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Adjust comments to stay with moved code

Just had a look at the heuristic in upstream and it _is_ using freeset available, so this change fixes a regression introduced only on the generational mode branch.

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PR: https://git.openjdk.org/shenandoah/pull/147