RFR: Make immediate trash of old regions with no live objects

[William Kemper]

At the end of old marking we can immediately recycle regions with no live objects. This optimization is already done for young/global collects, but it was missing from old collections. This fixes an assertion during preparation for evacuation that _all_ cset regions have _some_ live data.

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Commit messages:
 - Make immediate trash of old regions with no live objects

Changes: https://git.openjdk.java.net/shenandoah/pull/57/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=57&range=00
  Stats: 7 lines in 1 file changed: 4 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/57.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/57/head:pull/57

PR: https://git.openjdk.java.net/shenandoah/pull/57