Clean up weak objects in optimized code.
This prepares for weak objects in IC stubs.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/172233003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19489 ce2b1a6d-e550-0410-aec6-3dcde31c8c00