RFR: 8336086: G1: Use one G1CardSet instance for all young regions

[Ivan Walulya]

Hi all,

Please review this change to assign a single G1CardSet to all young regions. As young regions are collected at the same, and we do not have young-to-young remembered sets, we can maintain a single G1CardSet for all young regions.

This reduces the memory overhead of the G1CardSets and the time taken to merge per region G1CardSets during GC pause.

Testing: Tier 1-5

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Commit messages:
 - Merge remote-tracking branch 'upstream/master' into YoungOnlyCardSet
 - cleanup
 - merge
 - Merge remote-tracking branch 'upstream/master' into YoungOnlyCardSet
 - init

Changes: https://git.openjdk.org/jdk/pull/20134/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20134&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336086
  Stats: 175 lines in 21 files changed: 142 ins; 10 del; 23 mod
  Patch: https://git.openjdk.org/jdk/pull/20134.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20134/head:pull/20134

PR: https://git.openjdk.org/jdk/pull/20134