RFR: Coalesce and fill unmarked objects during global degenerated cycle

[William Kemper]

This is necessary to prevent the remembered set scan from iterating over garbage. We already do this for _concurrent_ global cycles, but it was missing from _degenerated _ global cycles.

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Commit messages:
 - Also coalesce and fill unmarked objects during global degenerated cycle

Changes: https://git.openjdk.java.net/shenandoah/pull/78/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=78&range=00
  Stats: 45 lines in 5 files changed: 28 ins; 16 del; 1 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/78.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/78/head:pull/78

PR: https://git.openjdk.java.net/shenandoah/pull/78