RFR: 8333893: Optimization for StringBuilder append boolean & null [v15]

[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.

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

  replace unsafe with putChar

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Changes:
  - all: https://git.openjdk.org/jdk/pull/19626/files
  - new: https://git.openjdk.org/jdk/pull/19626/files/7563577c..9d9c8eb8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19626&range=14
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19626&range=13-14

  Stats: 27 lines in 1 file changed: 0 ins; 18 del; 9 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