RFR: Combine bitmap clearing with region resetting closure

[Kelvin Nilsen]

On Tue, 31 Jan 2023 17:23:48 GMT, William Kemper <wkemper at openjdk.org> wrote:

> This change combines the two closures used to `prepare_gc`. This removes a second iteration over the regions.

Marked as reviewed by kdnilsen (Committer).

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PR: https://git.openjdk.org/shenandoah/pull/208