Stop using cfg.postorder_ebbs().

Switch to the new domtree.cfg_postorder() which returns a reference to a
pre-computed post-order instead of allocating memory and computing a new
post-order.

lib/cretonne/src/context.rs

@@ -75,7 +75,7 @@ impl Context {
 
     /// Run the legalizer for `isa` on the function.
     pub fn legalize(&mut self, isa: &TargetIsa) -> CtonResult {
-        legalize_function(&mut self.func, &mut self.cfg, isa);
+        legalize_function(&mut self.func, &mut self.cfg, &self.domtree, isa);
         self.verify_if(isa)
     }