RFR: Enforce that generation sizes align with region sizes

[Kelvin Nilsen]

For correctness, the size of each generation should be a multiple of the region size.

A recent change violated this requirement.

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Commit messages:
 - Enforce that generation sizes align with region sizes

Changes: https://git.openjdk.org/shenandoah/pull/191/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=191&range=00
  Stats: 5 lines in 2 files changed: 5 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/191.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/191/head:pull/191

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