Make sure value is a heap number when reusing the double box in BinaryOpICStub.

BUG=
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/216823005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20501 ce2b1a6d-e550-0410-aec6-3dcde31c8c00