RFR: 8341042: GenShen: Reset mark bitmaps for unaffiliated regions when preparing for a cycle

[William Kemper]

A recent change to reset unaffiliated bitmaps during the concurrent cleanup phase has caused a performance regression. Likely because concurrent cleanup only uses one thread (at this time). This issue is to revert the regression causing change while we work on a preferred solution.

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Commit messages:
 - Reset bitmaps for unaffiliated regions in prepare_gc

Changes: https://git.openjdk.org/shenandoah/pull/504/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=504&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341042
  Stats: 7 lines in 2 files changed: 1 ins; 5 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/504.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/504/head:pull/504

PR: https://git.openjdk.org/shenandoah/pull/504