RFR: 8336856: Optimize String Concat [v10]

[Shaojin Wen]

> The current implementation of StringConcat is to mix the coder and length into a long. This operation will have some overhead for int/long/boolean types. We can separate the calculation of the coder from the calculation of the length, which can improve the performance in the scenario of concat int/long/boolean.
> 
> This idea comes from the suggestion of @l4es in the discussion of PR https://github.com/openjdk/jdk/pull/20253#issuecomment-2240412866

Shaojin Wen has updated the pull request incrementally with ten additional commits since the last revision:

 - minor refactor
 - minor refactor
 - reduce change
 - copyright
 - reduce change
 - refactor based on 8335182
 - use Float.toString & Double.toString
 - remove ForceInline
 - fix build error

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/20273/files
  - new: https://git.openjdk.org/jdk/pull/20273/files/b274839c..c53ce54d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=09
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=08-09

  Stats: 385 lines in 4 files changed: 30 ins; 291 del; 64 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