RFR: 8334342: Add MergeStore JMH benchmarks [v7]

[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.

Shaojin Wen has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 16 additional commits since the last revision:

 - Merge remote-tracking branch 'upstream/master' into merge_store_bench
 - move to vm.compiler
 - Merge remote-tracking branch 'upstream/master' into merge_store_bench
 - Merge remote-tracking branch 'upstream/master' into merge_store_bench
 - bug fix for `putChars4C`
 - bug fix for `putChars4C` and `putChars4S`
 - use VarHandler CHAR_L & CHAR_B
 - copyright
 - bug fix for putIntBU
 - add cases for `getChar` & `putChar`
 - ... and 6 more: https://git.openjdk.org/jdk/compare/116db649...d00654ff

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/19734/files
  - new: https://git.openjdk.org/jdk/pull/19734/files/74f8837c..d00654ff

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19734&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19734&range=05-06

  Stats: 5554 lines in 236 files changed: 3846 ins; 825 del; 883 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