Add EmitKeyedStore and EmitKeyedPropertyAssignment to x64.  Removal of Reference class from VisitAssignment complete on all platforms.
Review URL: http://codereview.chromium.org/3028015

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5112 ce2b1a6d-e550-0410-aec6-3dcde31c8c00