RFR: 8336831: Optimize StringConcatHelper.simpleConcat [v6]

[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.

Shaojin Wen has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains ten commits:

 - merge from master
 - Merge remote-tracking branch 'upstream/master' into optim_simple_concat_202407
   
   # Conflicts:
   #	src/java.base/share/classes/java/lang/StringConcatHelper.java
 - Update src/java.base/share/classes/java/lang/String.java
   
   Co-authored-by: Chen Liang <liach at openjdk.org>
 - add comments
 - handle null argument
 - extract a common doConcat handling both non-empty strings
 - share newArray
 - copyright 2024
 - optimize String.concat

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Changes: https://git.openjdk.org/jdk/pull/20253/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20253&range=05
  Stats: 36 lines in 2 files changed: 20 ins; 0 del; 16 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