RFR: 8336856: Optimize String Concat [v17]

[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 with a new target base due to a merge or a rebase. The pull request now contains 44 commits:

 - fix mix Integer null error
 - fix stringSize build error
 - Merge remote-tracking branch 'upstream/master' into optim_concat_factory_202407
 - mix & prepend support Boolean/Integer/Long
 - more micro benchmark
 - rename simpleConcat -> makeSimpleConcat
 - remove benchmark
 - common simpleConcat
 - add benchmark
 - Descending mixer, same order as prepend
 - ... and 34 more: https://git.openjdk.org/jdk/compare/2fbdbaca...f241462c

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Changes: https://git.openjdk.org/jdk/pull/20273/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20273&range=16
  Stats: 716 lines in 5 files changed: 583 ins; 42 del; 91 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