RFR: Enforce max regions

[Kelvin Nilsen]

On Tue, 6 Dec 2022 17:57:18 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

> This commit enforces upper bounds on the number of ShenandoahHeapRegions affiliated with each generation.  Prior to this change, enforcement of generation sizes was by usage alone.  This allowed situations in which so many sparsely populated regions were affiliated with old-gen that there were insufficient FREE regions available to satisfy legitimate young-gen allocation requests.  This was resulting in excessive TLAB allocation failures and degenerated collections.

I'll also introduce a new method adjusted_unaffiliated_regions() to consolidate the code for the "common pattern" you identified.

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PR: https://git.openjdk.org/shenandoah/pull/179