RFR: 8339695: GenShen: Concurrent reset performance has regressed

[William Kemper]

A misguided optimization inadvertently increased the time to perform the concurrent reset phase of the GC cycle. This "optimization" has been reverted and the time for the phase has returned to normal.

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Commit messages:
 - Use region iterator for resetting bitmaps
 - Decouple mark bitmap reset from capturing TAMS closure

Changes: https://git.openjdk.org/shenandoah/pull/495/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=495&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339695
  Stats: 30 lines in 1 file changed: 10 ins; 10 del; 10 mod
  Patch: https://git.openjdk.org/shenandoah/pull/495.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/495/head:pull/495

PR: https://git.openjdk.org/shenandoah/pull/495