Reviewed by Gavin Barraclough.

        - Concatenate final three strings in simple replace case at one go

        ~0.2% SunSpider speedup

        * runtime/StringPrototype.cpp:
        (JSC::stringProtoFuncReplace): Use new replaceRange helper instead of
        taking substrings and concatenating three strings.
        * runtime/UString.cpp:
        (JSC::UString::replaceRange): New helper function.
        * runtime/UString.h:



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@43104 268f45cc-cd09-0410-ab3c-d52691b4dbfc