RFR: 8275055: Improve HeapRegionRemSet::split_card()

[Thomas Schatzl]

Hi all,

  can I have reviews for this small change that improves HeapRegionRemSet::split_card() by reducing the number of (direct and indirect) memory accesses for it. It is hard to actually measure improvements because it's only called in concurrent code. So no particular improvements measured.

It also prepares that method for virtualizing the remembered set containers, allowing arbitrarily large heap region sizes [JDK-8275056](https://bugs.openjdk.java.net/browse/JDK-8275056). This change is required then because the splitting of a card index into "region" and "card within region" should not be dependent on *heap regions*.

Testing: gha, gc/g1 local testing.

Thanks,
  Thomas

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Commit messages:
 - Optimize HeapRegionRemSet::split_card

Changes: https://git.openjdk.java.net/jdk/pull/5895/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5895&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275055
  Stats: 45 lines in 6 files changed: 42 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5895.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5895/head:pull/5895

PR: https://git.openjdk.java.net/jdk/pull/5895