If the input of a ++ or -- operation is a smi, the
result is an Integer32. Otherwise it is a number.

Review URL: http://codereview.chromium.org/1075004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4177 ce2b1a6d-e550-0410-aec6-3dcde31c8c00