RFR: 8364650: G1: Use InvalidCSetIndex instead of UINT_MAX for "invalid" sentinel value of young_index_in_cset

[Thomas Schatzl]

Hi all,

  please review this small change to use `InvalidCSetIndex` instead of hardcoded `UINT_MAX` for `G1HeapRegion::_young_index_in_cset`. Just reuse the existing value that is already in use for `_index_in_opt_cset`.

Testing: compilation, gha

Thanks,
  Thomas

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Commit messages:
 - Hi all,

Changes: https://git.openjdk.org/jdk/pull/26651/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26651&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364650
  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/26651.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26651/head:pull/26651

PR: https://git.openjdk.org/jdk/pull/26651