RFR: 8336856: Optimize String Concat [v9]

[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:

  refactor based on 8335182

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/20273/files
  - new: https://git.openjdk.org/jdk/pull/20273/files/d9d493a0..b274839c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=08
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=07-08

  Stats: 294 lines in 2 files changed: 178 ins; 57 del; 59 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