Visitor stack overflow is used to signal an unsupported construct in
the flow graph.  Check for it in more places.  Make the utility
functions for appending to graphs handle more cases if they can be
handled correctly.

Remove the entry node in favor of a block with a NULL predecessor as
single entry.  Represent the empty flow graph as a single empty block.
Add empty blocks lazily where needed to preserve edge-split form.

Review URL: http://codereview.chromium.org/804003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4086 ce2b1a6d-e550-0410-aec6-3dcde31c8c00