Omit smi check inside write barriers if the value is known to be a heap object.

Refine inferred types of some instructions.
Review URL: http://codereview.chromium.org/8256016

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9618 ce2b1a6d-e550-0410-aec6-3dcde31c8c00