Remove unneeded checks for SSE2 in X64 code.
Remove an unneeded check for SSE3 where the X64 code doesn't use SSE3 features.

Review URL: http://codereview.chromium.org/6793040

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7508 ce2b1a6d-e550-0410-aec6-3dcde31c8c00