The flow graph is built by walking the AST.  Edges are labeled with
instructions (AST nodes).  Normal nodes have a single predecessor edge and a
single (labeled) successor edge.  Branch nodes are explicit, they have a
single predecessor edge and a pair of (unlabeled) successor edges.  Merge
nodes are explicit, they have a pair of predecessor edges and a single
(unlabeled) successor edge.

There is a distinguished (normal) entry node and a distinguished (special)
exit node with arbitrarily many predecessor edges and no successor edges.

The graph is intended to support graph-based analysis and transformation.

Review URL: http://codereview.chromium.org/660449

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4051 ce2b1a6d-e550-0410-aec6-3dcde31c8c00