RFR: 8336856: Optimize String Concat

[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

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Depends on: https://git.openjdk.org/jdk/pull/20253

Commit messages:
 - optimize string concat

Changes: https://git.openjdk.org/jdk/pull/20273/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336856
  Stats: 393 lines in 5 files changed: 117 ins; 76 del; 200 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