This change adds a post-order numbering to AST nodes that
are relevant for the fast code generator. It is only invoked
together with the fast compiler. 

Also changed the ast printer to print the  numbering for 
testing purposes if it is present.

Review URL: http://codereview.chromium.org/553134

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3738 ce2b1a6d-e550-0410-aec6-3dcde31c8c00