RFR: Enforce that generation sizes align with region sizes

[Y. Srinivas Ramakrishna]

On Wed, 4 Jan 2023 16:12:35 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

> For correctness, the size of each generation should be a multiple of the region size.
> 
> A recent change violated this requirement.

LGTM.

Please feel free to include testing notes, if any. Thanks!

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Marked as reviewed by ysr (Author).

PR: https://git.openjdk.org/shenandoah/pull/191