RFR: 8336856: Optimize String Concat [v28]

[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 11 additional commits since the last revision:

 - 1. split length & coder & prepend method
   2. fix comment
   3. change forceInlineThreshold to 16
   4. 1 arg use built-in method
   5. add tests
 - Extract an erasedArgs method
 - Remove unnecessary operations
 - copyright
 - typo
 - bug fix and make clean
 - Code structuring to reduce the size of generated code
 - forceinline
 - use prepend and split lengthcoder
 - Revert "use static method, remove ConcatBase & MethodHandlePair & CONSTRUCTOR_BUILDER"
   
   This reverts commit 03d0c3e6d638a94b29000da510e3b69abdbbe240.
 - ... and 1 more: https://git.openjdk.org/jdk/compare/a3cd822e...937f4117

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20273/files
  - new: https://git.openjdk.org/jdk/pull/20273/files/a3cd822e..937f4117

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=27
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=26-27

  Stats: 811 lines in 5 files changed: 512 ins; 132 del; 167 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