We now test for a smi before calling ToNumber and inline
the smi increment/decrement for ++ and --. There only a small
increase in code size but loops in top-level code are
becoming much faster as a result.

Review URL: http://codereview.chromium.org/553056

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3681 ce2b1a6d-e550-0410-aec6-3dcde31c8c00