RFR: Update card table when cycle degenerates during root scan

[William Kemper]

If the concurrent cycle degenerates during the root scan (after the card table has been swapped), we need to update the _read_ table (used during the STW mark) with any cards written during the transition to the degenerated cycle. Without this change, the remembered set scan on the STW mark will skip old to young pointers created during the transition.

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Commit messages:
 - Merge dirty write cards when cycle degenerates during root scan

Changes: https://git.openjdk.java.net/shenandoah/pull/106/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=106&range=00
  Stats: 59 lines in 4 files changed: 59 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/106.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/106/head:pull/106

PR: https://git.openjdk.java.net/shenandoah/pull/106