Cleanup the AST code by removing unused parts and get rid of the

Cleanup the AST code by removing unused parts and get rid of the
flow graph code completely. Add new AST node for null compares
and use it to make the full codegens better at generating code
for null comparisons.
Review URL: http://codereview.chromium.org/3146037

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5323 ce2b1a6d-e550-0410-aec6-3dcde31c8c00