RFR: 8336856: Optimize String Concat [v2]

[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:

  non-MH-based implementation

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20273/files
  - new: https://git.openjdk.org/jdk/pull/20273/files/227158a7..c9acbe93

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=00-01

  Stats: 897 lines in 4 files changed: 206 ins; 686 del; 5 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