Improve replaceUsingStringSearch() for case of a single character searchValue

Improve replaceUsingStringSearch() for case of a single character searchValue
https://bugs.webkit.org/show_bug.cgi?id=83738

Patch by Benjamin Poulain <bpoulain@apple.com> on 2012-04-12
Reviewed by Geoffrey Garen.

This patch improves replaceUsingStringSearch() with the following:
-Add a special case for single character search, taking advantage of the faster WTF::find().
-Inline replaceUsingStringSearch().
-Use StringImpl::create() instead of UString::substringSharingImpl() since we know we are in the bounds
 by definition.

This gives less than 1% improvement for the multicharacter replace.
The single character search show about 9% improvement.

* runtime/StringPrototype.cpp:
(JSC::replaceUsingStringSearch):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@114072 268f45cc-cd09-0410-ab3c-d52691b4dbfc