RFR: 8272231 G1: Refactor G1CardSet::get_card_set to return G1CardSetHashTableValue*

[Ivan Walulya]

Hi all,

Please review this change to return G1CardSetHashTableValue* from G1CardSet::get_card_set. Consequently, we do not have to use G1CardSet::get_or_add_card_set in instances where we only need to get the table entry (such as in G1CardSet::transfer_cards_in_howl).

Testing: Tier 1-5.

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Commit messages:
 - refactor G1CardSet::get_card_set

Changes: https://git.openjdk.java.net/jdk/pull/5072/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5072&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272231
  Stats: 22 lines in 2 files changed: 6 ins; 3 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5072.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5072/head:pull/5072

PR: https://git.openjdk.java.net/jdk/pull/5072