RFR: 8314100: G1: Improve collection set candidate selection code

[Thomas Schatzl]

Hi all,

  discussions during review of [JDK-8140326](https://bugs.openjdk.org/browse/JDK-8140326) indicated that he code surrounding selecting candidates after marking/rebuild for the collection set could be improved.

Testing: gha

Thanks,
  Thomas

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Commit messages:
 - initial version

Changes: https://git.openjdk.org/jdk/pull/15243/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15243&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314100
  Stats: 43 lines in 2 files changed: 12 ins; 24 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/15243.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15243/head:pull/15243

PR: https://git.openjdk.org/jdk/pull/15243