RFR: JDK-8262994: Refactor String.split to help method inlining

[Christian Wimmer]

The method `String.split` contains a fast-path when the regular expression parameter is not really a regular expression, but just a single split character.
This fast path vs. slow path check can be constant folded when the regular expression parameter is a literal constant - a quite frequent pattern (for example, all JDK usages of `String.split` have a constant expression parameter). But method inlining in JIT and AOT compilers can usually not inline `String.split` because the method body is too large. Factoring out the actual fast-path splitting logic into a separate method solves this problem: the JIT or AOT compiler can inline `String.split`, constant-fold the fast/slow path check, and then only the invoke of either the fast path or the slow path remains.

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Commit messages:
 - JDK-8262994: Refactor String.split to help method inlining

Changes: https://git.openjdk.org/jdk/pull/11791/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11791&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8262994
  Stats: 40 lines in 1 file changed: 8 ins; 2 del; 30 mod
  Patch: https://git.openjdk.org/jdk/pull/11791.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11791/head:pull/11791

PR: https://git.openjdk.org/jdk/pull/11791