RFR: 8336856: Optimize String Concat [v6]

[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:

  1. reduce change
  2. add flag GENERATE_INLINE_COPY

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20273/files
  - new: https://git.openjdk.org/jdk/pull/20273/files/54744017..4c2a96fc

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=04-05

  Stats: 907 lines in 5 files changed: 761 ins; 35 del; 111 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