RFR: 8334342: Add MergeStore JMH benchmarks

[Shaojin Wen]

[8318446](https://github.com/openjdk/jdk/pull/16245)  brings MergeStore. We need a JMH Benchmark to evaluate the performance of various batch operations and the effect of MergeStore.

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Commit messages:
 - code format
 - add `setIntRL` & `setIntRLU`
 - add comments
 - add comments
 - copyright
 - add MergeStore Benchmark

Changes: https://git.openjdk.org/jdk/pull/19734/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19734&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334342
  Stats: 870 lines in 1 file changed: 870 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/19734.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19734/head:pull/19734

PR: https://git.openjdk.org/jdk/pull/19734