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

[Kelvin Nilsen]

On Wed, 10 May 2023 20:39:49 GMT, William Kemper <wkemper at openjdk.org> wrote:

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

Marked as reviewed by kdnilsen (Committer).

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PR Review: https://git.openjdk.org/shenandoah/pull/278#pullrequestreview-1421399369