In case one of the arguments is known to be a string we emit a few
fast conversion attempts for the other.  This allows using the
StringAddStub instead of STRING_ADD_{LEFT,RIGHT} builtins.

Review URL: http://codereview.chromium.org/3338007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5405 ce2b1a6d-e550-0410-aec6-3dcde31c8c00