RFR: Combine bitmap clearing with region resetting closure

[William Kemper]

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

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Commit messages:
 - Combine bitmap clearing with region resetting closure

Changes: https://git.openjdk.org/shenandoah/pull/208/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=208&range=00
  Stats: 14 lines in 1 file changed: 8 ins; 1 del; 5 mod
  Patch: https://git.openjdk.org/shenandoah/pull/208.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/208/head:pull/208

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