RFR: Coalesce and fill dead objects after class unloading

[William Kemper]

This reverts a recent change to coalesce and fill dead objects immediately after final mark. These objects cannot be filled until after class unloading to support a use case where the LRB needs to access from-space objects. So, rather than coalesce and fill during the concurrent mark phase, we move it to the evacuation phase of a degenerated cycle.

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Commit messages:
 - Coalesce and fill dead objects after class unloading

Changes: https://git.openjdk.java.net/shenandoah/pull/111/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=111&range=00
  Stats: 15 lines in 2 files changed: 8 ins; 7 del; 0 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/111.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/111/head:pull/111

PR: https://git.openjdk.java.net/shenandoah/pull/111