RFR: 8314100: G1: Improve collection set candidate selection code [v2]

[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

Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:

  Improve code after ayang review

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/15243/files
  - new: https://git.openjdk.org/jdk/pull/15243/files/b0b5f89c..40cf71dc

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15243&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15243&range=00-01

  Stats: 18 lines in 1 file changed: 9 ins; 4 del; 5 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