RFR: Abandon old gen cset candidates upon global collection

[William Kemper]

When a _global_ collection occurs, it will include the old generation. When this happens, it doesn't make sense to continue with mixed collections of the old generation. This changes fixes an assert caused by the same old region being added more than once to the collection set.

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Commit messages:
 - Abandon old gen cset candidates upon global collection

Changes: https://git.openjdk.java.net/shenandoah/pull/38/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=38&range=00
  Stats: 31 lines in 7 files changed: 30 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/38.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/38/head:pull/38

PR: https://git.openjdk.java.net/shenandoah/pull/38