RFR: 8336856: Optimize String Concat [v30]

[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 with a new target base due to a merge or a rebase. The pull request now contains 93 commits:

 - Merge remote-tracking branch 'upstream/master' into optim_concat_factory_202408
 - revert simplify prepend
 - simplify prepend & code style
 - Update src/java.base/share/classes/java/lang/invoke/StringConcatFactory.java
   
   Co-authored-by: David Schlosnagle <schlosna at gmail.com>
 - Update src/java.base/share/classes/java/lang/StringConcatHelper.java
   
   Co-authored-by: David Schlosnagle <schlosna at gmail.com>
 - copyright
 - add comments & code style & make code clear
 - add comments and make code clear
 - fix comments and reduce code size of generated methods
 - 1. split length & coder & prepend method
   2. fix comment
   3. change forceInlineThreshold to 16
   4. 1 arg use built-in method
   5. add tests
 - ... and 83 more: https://git.openjdk.org/jdk/compare/be34730f...726b9ad8

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Changes: https://git.openjdk.org/jdk/pull/20273/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=29
  Stats: 1259 lines in 7 files changed: 1135 ins; 32 del; 92 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