RFR: 6714245: [Col] Collator - Faster Comparison for identical strings.

[Justin Lu]

Please review this PR which adds an initial equality check to `RuleBasedCollator.compare(String source, String target)`.

This speeds up the operation for equal input Strings, as it bypasses Collator rule comparisons of each element for both of the input Strings.

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Commit messages:
 - Check for equality first in compare(string, string)

Changes: https://git.openjdk.org/jdk/pull/13938/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13938&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-6714245
  Stats: 5 lines in 1 file changed: 4 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13938.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13938/head:pull/13938

PR: https://git.openjdk.org/jdk/pull/13938