RFR: 8336831: Optimize StringConcatHelper.simpleConcat [v3]

[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 one additional commit since the last revision:

  extract a common doConcat handling both non-empty strings

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20253/files
  - new: https://git.openjdk.org/jdk/pull/20253/files/38a6f560..a509b7cd

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20253&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20253&range=01-02

  Stats: 6 lines in 2 files changed: 5 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