Don't form identifiers the first time a string is used as a property name.

Reviewed by Oliver Hunt.

This is a 1% win on SunSpider.

* dfg/DFGOperations.cpp:
    - Use fastGetOwnProperty.
* jit/JITStubs.cpp:
(JSC::DEFINE_STUB_FUNCTION):
    - Use fastGetOwnProperty.
* runtime/JSCell.h:
* runtime/JSObject.h:
(JSC::JSCell::fastGetOwnProperty):
    - Fast call to get a property without creating an identifier the first time.
* runtime/PropertyMapHashTable.h:
(JSC::PropertyTable::find):
(JSC::PropertyTable::findWithString):
    - Add interface to look up by either strinsg or identifiers.
* runtime/Structure.h:
(JSC::Structure::get):
    - Add a get() call that takes a UString, not an Identifier.
* wtf/text/StringImpl.h:
(WTF::StringImpl::hasHash):
    - Add a call to check if the has has been set (to detect the first use as a property name).



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@92569 268f45cc-cd09-0410-ab3c-d52691b4dbfc