AstNode in case we ever want to have some other kind of node.

Split the NODE_LIST macro-generating macro so that we can iterate
concrete subclasses of Statement and concrete subclasses of Expression
separately.
Review URL: http://codereview.chromium.org/159632

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2586 ce2b1a6d-e550-0410-aec6-3dcde31c8c00