[iOS] Make a UChar string equal() based on the LChar version
https://bugs.webkit.org/show_bug.cgi?id=112495

Reviewed by David Kilzer.

* wtf/text/StringImpl.h:
(WTF::equal):
Create a equal() function for UChar based on the work done for LChar.

On A6, this is a speed up of about 40% for any string of 2 or more characters.
It is slower by 8% on a single UChar comparison.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@146170 268f45cc-cd09-0410-ab3c-d52691b4dbfc