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.

src/print_cfg.rs

@@ -33,7 +33,7 @@ impl<'a> CFGPrinter<'a> {
     pub fn new(func: &'a Function) -> CFGPrinter<'a> {
         CFGPrinter {
             func: func,
-            cfg: ControlFlowGraph::new(func),
+            cfg: ControlFlowGraph::with_function(func),
         }
     }