RFR: Use whole number of regions when resizing generations

[William Kemper]

This avoids overflowing calculations when using a 32 bit word - it also simplifies some of the operations. All of the github actions are succeeding now.

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Commit messages:
 - Merge branch 'openjdk:master' into use-regions-for-sizing
 - Use region count rather than bytes count to avoid overflow with 32 bit words

Changes: https://git.openjdk.org/shenandoah/pull/199/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=199&range=00
  Stats: 76 lines in 2 files changed: 15 ins; 5 del; 56 mod
  Patch: https://git.openjdk.org/shenandoah/pull/199.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/199/head:pull/199

PR: https://git.openjdk.org/shenandoah/pull/199