RFR: Add generations to freeset [v20]

[Kelvin Nilsen]

On Fri, 28 Apr 2023 15:36:23 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> ShenandoahFreeSet has not yet been modified to deal efficiently with the combination of old-gen and young-gen collection set reserves.  This PR makes changes so that we can distinguish between collector_is_free, old_collector_is_free, and mutator_is_free.  Further, it endeavors to keep each set of free regions tightly packed, so the range of regions representing each set is small.
>> 
>> In its current form, this no longer fails existing regression tests (except for known problems that are being addressed independently)
>
> Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix white space

It occurs to me that this is a "big stretch" for a single PR.  It may be preferable to integrate this in two phases:
1. The first phase could commit up to: https://github.com/openjdk/shenandoah/pull/250/commits/202ee74766e31ebaba3d87770ff40bb40ccbea58
2. The second phase, could integrate the refactoring that followed that stable commit

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PR Comment: https://git.openjdk.org/shenandoah/pull/250#issuecomment-1528181012