RFR: Refactor budgeting to make the logic cleaner [v6]

[William Kemper]

On Wed, 13 Jul 2022 16:33:03 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> This commit consolidates computations related to budgeting of reserves for old-gen evacuation and old-gen promotion within shenandoahGeneration::prepare_regions_and_collection_set().  Previously, these computations had been scattered between several functional units, including shenandoahHeuristics::choose_collection_set() and shenandoahOldHeuristics::prime_collection_set().  A previous pull request brought some of the redundant functionality into prepare_regions_and_collection_set().  This pull request removes the redundant computations from the other locations.
>
> Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Make helper methods private

Marked as reviewed by wkemper (Committer).

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PR: https://git.openjdk.org/shenandoah/pull/148