RFR: Assert that region usage accounting is only accessed in generational mode

[William Kemper]

There was one unrelated test failure in the github actions (running G1).

-------------

Commit messages:
 - Use gerund verb form for assert message
 - Merge remote-tracking branch 'shenandoah/master' into add-used-regions-assert
 - Assert that used_regions is only accessed for generational mode

Changes: https://git.openjdk.org/shenandoah/pull/278/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=278&range=00
  Stats: 2 lines in 1 file changed: 1 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/278.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/278/head:pull/278

PR: https://git.openjdk.org/shenandoah/pull/278