Make the use of xmm0 as double scratch register explicit in ia32 and x64.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/24277002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16847 ce2b1a6d-e550-0410-aec6-3dcde31c8c00