RFR (XS) 8060485: (str) contentEquals checks the String contents twice on mismatch

[Aleksey Shipilev]

Hi,

Please review a trivial change in String.contentEquals:
  https://bugs.openjdk.java.net/browse/JDK-8060485
  http://cr.openjdk.java.net/~shade/8060485/webrev.00/

It improves the performance drastically:
  http://cr.openjdk.java.net/~shade/8060485/perf.txt

...not to mention it improves the code readability, and protects us from
rogue CharSequence-s.

Testing: microbenchmarks, jdk/test/String* jtreg.

Thanks,
-Aleksey.