RFR: 8336856: Optimize String Concat [v27]

[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:

  remove lengthcoder

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20273/files
  - new: https://git.openjdk.org/jdk/pull/20273/files/c91b82a8..a3cd822e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=26
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=25-26

  Stats: 452 lines in 2 files changed: 296 ins; 74 del; 82 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