[turbofan] Correctly use bit_cast instead of static_cast.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/559913003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23825 ce2b1a6d-e550-0410-aec6-3dcde31c8c00