Track whether a node or variable are likely to be a Smi value. Propagate that

Track whether a node or variable are likely to be a Smi value. Propagate that
knowledge in the AST and inline the Smi check into the generated code if it
is deemed high value (e.g. in loops).

Review URL: http://codereview.chromium.org/8835

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@630 ce2b1a6d-e550-0410-aec6-3dcde31c8c00