Inlined the fast path for creating an Identifier from an Identifier. 
        
        This is a .4% speedup on SunSpider overall, but as big as a 2.5% 
        speedup on certain individual tests. 65% of the Identifiers creating 
        by SunSpider are already Identifiers.
        
        (The main reason I'm making this change is that it resolves a large
        regression in a patch I haven't checked in yet.)

        * JavaScriptCore.exp:
        * kjs/identifier.cpp:
        (KJS::Identifier::addSlowCase):
        * kjs/identifier.h:
        (KJS::Identifier::Identifier::add):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@26994 268f45cc-cd09-0410-ab3c-d52691b4dbfc