RFR: 8328361: Use 'memset' in method 'CardTable::dirty_MemRegion'

[Guoxiong Li]

Hi all,

This patch uses `memset` instead of explicit loop in method `CardTable::dirty_MemRegion` to simplify the code. Thanks for taking the time to review.

Best Regards,
-- Guoxiong

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Commit messages:
 - Use memset instead of loop.

Changes: https://git.openjdk.org/jdk/pull/18349/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18349&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328361
  Stats: 4 lines in 1 file changed: 0 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18349.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18349/head:pull/18349

PR: https://git.openjdk.org/jdk/pull/18349