TryFlatten is inlined, while Flatten is not.  Make an optimization to avoid the call to Flatten when we're already flat.  This gives me 5% on some simple indexOf experiments.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@347 ce2b1a6d-e550-0410-aec6-3dcde31c8c00