RFR: 8367743: G1: Use named constants for G1CSetCandidateGroup group ids

[Thomas Schatzl]

Hi all,

  please review this refactoring to use named constants for cset candidate group ids instead of raw integers.

Testing: gha, local compilation, use in another patch for weeks

Thanks,
  Thomas

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Commit messages:
 - 8367743

Changes: https://git.openjdk.org/jdk/pull/27309/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27309&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367743
  Stats: 21 lines in 4 files changed: 9 ins; 2 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/27309.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27309/head:pull/27309

PR: https://git.openjdk.org/jdk/pull/27309