RFR: 8374780: G1: Do not iterate klass metadata when splitting objArrays for every slice

[Thomas Schatzl]

Hi all,

  please review this change that makes concurrent mark not try to iterate over `objArray` metadata for every slice, to make behavior uniform with other garbage collection types/garbage collectors.

Testing: tier1-5

Thanks,
  Thomas

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Commit messages:
 - 8374780

Changes: https://git.openjdk.org/jdk/pull/29116/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29116&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8374780
  Stats: 22 lines in 5 files changed: 12 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/29116.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29116/head:pull/29116

PR: https://git.openjdk.org/jdk/pull/29116