eliminate PIC branches by changing NaN handling in JSValue::toNumber

Reviewed by Sam Weinig.

Moving the non-numeric cases out of line seems to be a consistent
win on SunSpider for me, to the order of about 0.5%.

* runtime/JSCell.h:
(JSC::JSCell::JSValue::toNumber):
    - Changed to only handle values that are already numbers, moce non-numeric cases out of line.
* runtime/JSValue.cpp:
(JSC::JSValue::toNumberSlowCase):
    - Added toNumberSlowCase, handling non-numeric cases.
* runtime/JSValue.h:
    - Add declaration of toNumberSlowCase.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@91555 268f45cc-cd09-0410-ab3c-d52691b4dbfc