RFR: 8340137: GenShen: Reset mark bitmaps when a region is recycled

[William Kemper]

Historically, the concurrent reset phase has reset bitmaps for _all_ committed regions. In the generational mode, however, we wish to reset bitmaps for only the generation being collected. This action was recently made to exclude free regions. This broke an assert for regions that became committed (active) after concurrent reset. To address this, we can reset bitmaps for a region when it is recycled (resets still happen for active regions during concurrent reset).

## Testing
GHA, internal pipelines

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Commit messages:
 - Clear bitmaps when region is recycled

Changes: https://git.openjdk.org/shenandoah/pull/500/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=500&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340137
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/500.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/500/head:pull/500

PR: https://git.openjdk.org/shenandoah/pull/500