RFR: 8365122: G1: Minor clean up of G1SurvivorRegions

[Thomas Schatzl]

Hi all,

  please review this small cleanup around `G1SurvivorRegions`:

- make the `_regions` member a direct member of the class
- use the C++ for-each construct for iterating over `_regions` makes the code more readable

Testing: gha

Thanks,
  Thomas

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Commit messages:
 - 8365122

Changes: https://git.openjdk.org/jdk/pull/26692/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26692&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365122
  Stats: 26 lines in 3 files changed: 1 ins; 10 del; 15 mod
  Patch: https://git.openjdk.org/jdk/pull/26692.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26692/head:pull/26692

PR: https://git.openjdk.org/jdk/pull/26692