RFR: Fix verification of remembered set at mark start

[Kelvin Nilsen]

All objects residing between TAMS and top() within each old region are examined independent of the marking context.

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Commit messages:
 - Fix verification of remembered set at mark start

Changes: https://git.openjdk.org/shenandoah/pull/197/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=197&range=00
  Stats: 46 lines in 1 file changed: 29 ins; 7 del; 10 mod
  Patch: https://git.openjdk.org/shenandoah/pull/197.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/197/head:pull/197

PR: https://git.openjdk.org/shenandoah/pull/197