Merged r13648 into 3.16 branch.

Don't try to unlink instructions twice during GVN

BUG=chrome:175141

R=danno@chromium.org

Review URL: https://codereview.chromium.org/12353002

git-svn-id: https://v8.googlecode.com/svn/branches/3.16@13746 ce2b1a6d-e550-0410-aec6-3dcde31c8c00