RFR: Allow adjusted capacity and used regions size to be equal

[William Kemper]

Fix assertion which requires adjusted capacity to be larger than the used regions size (they may be equal).

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Commit messages:
 - Allow adjusted capacity and used regions size to be equal

Changes: https://git.openjdk.org/shenandoah/pull/181/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=181&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/181.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/181/head:pull/181

PR: https://git.openjdk.org/shenandoah/pull/181