RFR: 8343629: More MergeStore benchmark [v5]

[Shaojin Wen]

> 1. Added the putBytes4 benchmark, which corresponds to StringBuilder appendNull
> 2. Optimized the putChars4/setInt/setLong series of benchmarks to reduce extra overhead and more accurately reflect performance differences.

Shaojin Wen has updated the pull request incrementally with one additional commit since the last revision:

  seperate MergeStoreBench and MergeLoadBench

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Changes:
  - all: https://git.openjdk.org/jdk/pull/21659/files
  - new: https://git.openjdk.org/jdk/pull/21659/files/2e88b024..dce66dae

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21659&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21659&range=03-04

  Stats: 943 lines in 2 files changed: 515 ins; 428 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/21659.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21659/head:pull/21659

PR: https://git.openjdk.org/jdk/pull/21659