RFR: 8336856: Optimize String Concat [v18]

[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 five additional commits since the last revision:

 - code style & add comments
 - Revert "add none-MH benchmark"
   
   This reverts commit 8abba288fb403aeb621016154453c6b3393033fa.
 - add none-MH benchmark
 - reduce change
 - remove mix & prepend Boolean/Integer/Long

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/20273/files
  - new: https://git.openjdk.org/jdk/pull/20273/files/f241462c..64ce59d2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=17
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=16-17

  Stats: 262 lines in 3 files changed: 4 ins; 163 del; 95 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