This is ok since we don't care about preserving the upper half
of xmm registers in Crankshaft code.
Review URL: http://codereview.chromium.org/6880007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7660 ce2b1a6d-e550-0410-aec6-3dcde31c8c00