RFR: Enforce that generation sizes align with region sizes

[Kelvin Nilsen]

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.

I discovered this problem because of an assertion failure in a separate branch that was based on mainline.  I added the same assertion into this branch and verified through our internal pipeline regression testing that the two corrections to existing implementation resolve the assertion failure and do not introduce any other regressions.

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