Fix invalid SMI test in x64 ToNumberStub::Generate().

TBR=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/19927002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15788 ce2b1a6d-e550-0410-aec6-3dcde31c8c00