RFR: 8336856: Optimize String Concat [v15]

[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 one additional commit since the last revision:

  add benchmark

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/20273/files
  - new: https://git.openjdk.org/jdk/pull/20273/files/e430a417..00fcd554

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=14
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=13-14

  Stats: 141 lines in 2 files changed: 141 ins; 0 del; 0 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