RFR: 8333893: Optimization for StringBuilder append boolean & null

[Shaojin Wen]

After PR https://github.com/openjdk/jdk/pull/16245, C2 optimizes stores into primitive arrays by combining values ​​into larger stores.

This PR rewrites the code of appendNull and append(boolean) methods so that these two methods can be optimized by C2.

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Commit messages:
 - Optimization for StringBuilder append(boolean) & appendNull

Changes: https://git.openjdk.org/jdk/pull/19626/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19626&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8333893
  Stats: 137 lines in 5 files changed: 54 ins; 41 del; 42 mod
  Patch: https://git.openjdk.org/jdk/pull/19626.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19626/head:pull/19626

PR: https://git.openjdk.org/jdk/pull/19626