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.

lib/cretonne/src/context.rs

@@ -4,12 +4,16 @@
 //! deallocating the data structures needed for compilation. The `Context` struct is used to hold
 //! on to memory allocations between function compilations.
 
+use cfg::ControlFlowGraph;
 use ir::Function;
 
 /// Persistent data structures and compilation pipeline.
 pub struct Context {
     /// The function we're compiling.
     pub func: Function,
+
+    /// The control flow graph of `func`.
+    pub cfg: ControlFlowGraph,
 }
 
 impl Context {
@@ -18,6 +22,9 @@ impl Context {
     /// The returned instance should be reused for compiling multiple functions in order to avoid
     /// needless allocator thrashing.
     pub fn new() -> Context {
-        Context { func: Function::new() }
+        Context {
+            func: Function::new(),
+            cfg: ControlFlowGraph::new(),
+        }
     }
 }