RFR: 8336831: Optimize StringConcatHelper.simpleConcat [v5]

[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 incrementally with two additional commits since the last revision:

 - Update src/java.base/share/classes/java/lang/String.java
   
   Co-authored-by: Chen Liang <liach at openjdk.org>
 - add comments

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20253/files
  - new: https://git.openjdk.org/jdk/pull/20253/files/9b39c4c7..69901157

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20253&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20253&range=03-04

  Stats: 11 lines in 2 files changed: 10 ins; 0 del; 1 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