Fix AstGraphBuilder for loops like for(;;).

R=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/640203004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24777 ce2b1a6d-e550-0410-aec6-3dcde31c8c00