RFR: 8284637: Improve String.join performance

[XenoAmess]

8284637: Improve String.join performance

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Commit messages:
 - refine performance of String#join

Changes: https://git.openjdk.java.net/jdk/pull/8169/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8169&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284637
  Stats: 21 lines in 1 file changed: 16 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8169.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8169/head:pull/8169

PR: https://git.openjdk.java.net/jdk/pull/8169