RFR: 8336856: Optimize String Concat [v41]

[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 two additional commits since the last revision:

 - fix comments
 - remove unused code

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20273/files
  - new: https://git.openjdk.org/jdk/pull/20273/files/3dd9e658..9beb8c6a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=40
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=39-40

  Stats: 9 lines in 1 file changed: 3 ins; 2 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