RFR: Mixed evacuation [v2]

[Kelvin Nilsen]

> Mixed evacuation

Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:

  Chasing bug during prepare_for_old_collection
  
  One bug fix, a few cosmetic improvements, a change in loop structure to possibly avoid a gcc
  optimization error.

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Changes:
  - all: https://git.openjdk.java.net/shenandoah/pull/29/files
  - new: https://git.openjdk.java.net/shenandoah/pull/29/files/7bbd6a37..50ffb169

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=29&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=29&range=00-01

  Stats: 27 lines in 3 files changed: 8 ins; 7 del; 12 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/29.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/29/head:pull/29

PR: https://git.openjdk.java.net/shenandoah/pull/29