RFR: 8336086: G1: Use one G1CardSet instance for all young regions [v3]

[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

Ivan Walulya has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains ten commits:

 - Thomas Review
 - Merge remote-tracking branch 'upstream/master' into YoungOnlyCardSet
 - Albert Review
 - Merge remote-tracking branch 'upstream/master' into YoungOnlyCardSet
 - 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=02
  Stats: 183 lines in 21 files changed: 150 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