RFR: 8336856: Optimize String Concat [v38]

[Shaojin Wen]

> This PR implements the same algorithm as the current generateMHInlineCopy based on bytecode to improve startup performance.

Shaojin Wen has updated the pull request incrementally with one additional commit since the last revision:

  fix comments

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20273/files
  - new: https://git.openjdk.org/jdk/pull/20273/files/b260874a..6895db8a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=37
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=36-37

  Stats: 8 lines in 1 file changed: 3 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/20273.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20273/head:pull/20273

PR: https://git.openjdk.org/jdk/pull/20273