Make the ControlFlowGraph reusable.

Move the flow graph computation into a compute method which can be
called with multiple functions.

This allows us to reuse the ControlFlowGraph memory and keep an instance
in the Context.

tests/cfg_traversal.rs

@@ -8,7 +8,7 @@ use self::cretonne::entity_map::EntityMap;
 
 fn test_reverse_postorder_traversal(function_source: &str, ebb_order: Vec<u32>) {
     let func = &parse_functions(function_source).unwrap()[0];
-    let cfg = ControlFlowGraph::new(&func);
+    let cfg = ControlFlowGraph::with_function(&func);
     let ebbs = ebb_order.iter()
         .map(|n| Ebb::with_number(*n).unwrap())
         .collect::<Vec<Ebb>>();