Moved initialization of xmm3 making sure it is initialized on all paths in GeneratePow - solves the sputnik failure on mac.

Review URL: http://codereview.chromium.org/661177

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3973 ce2b1a6d-e550-0410-aec6-3dcde31c8c00