RFR: 8336831: Optimize StringConcatHelper.simpleConcat

[Shaojin Wen]

Currently simpleConcat is implemented using mix and prepend, but in this simple scenario, it can be implemented in a simpler way and can improve performance.

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Commit messages:
 - copyright 2024
 - optimize String.concat

Changes: https://git.openjdk.org/jdk/pull/20253/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20253&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336831
  Stats: 11 lines in 1 file changed: 0 ins; 4 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/20253.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20253/head:pull/20253

PR: https://git.openjdk.org/jdk/pull/20253