RFR: 8336856: Optimize String Concat [v29]

[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 four additional commits since the last revision:

 - copyright
 - add comments & code style & make code clear
 - add comments and make code clear
 - fix comments and reduce code size of generated methods

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20273/files
  - new: https://git.openjdk.org/jdk/pull/20273/files/937f4117..520f4ced

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=28
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=27-28

  Stats: 244 lines in 3 files changed: 114 ins; 68 del; 62 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