RFR: 8336856: Optimize String Concat [v16]

[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 two additional commits since the last revision:

 - remove benchmark
 - common simpleConcat

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/20273/files
  - new: https://git.openjdk.org/jdk/pull/20273/files/00fcd554..f1898af9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=15
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=14-15

  Stats: 175 lines in 3 files changed: 20 ins; 149 del; 6 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