They do not use subgraphs or subgraph scopes.  Instead of computing a list
of single-block subgraphs and then adding all the edges afterward, build
both the blocks and edges directly.

Review URL: http://codereview.chromium.org/6615014

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7058 ce2b1a6d-e550-0410-aec6-3dcde31c8c00