Skip write barrier if value and object originate from the same allocation.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/101993002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18243 ce2b1a6d-e550-0410-aec6-3dcde31c8c00