Drop the domtree argument to legalize_function().

Future legalization patterns will have the ability to mutate the
flowgraph, so the domtree's list of RPO blocks is not a good guide for
iteration. Use the layout order instead. This will pick up any new EBBs
inserted.

lib/cretonne/src/context.rs

@@ -103,7 +103,7 @@ 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, &self.domtree, isa);
+        legalize_function(&mut self.func, &mut self.cfg, isa);
         self.verify_if(isa)
     }