Optimize the emitted instruction of random function for X64

Review URL: https://codereview.chromium.org/11852007
Patch from Weiliang Lin <weiliang.lin@intel.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13393 ce2b1a6d-e550-0410-aec6-3dcde31c8c00