Change dfg.inst_results to return a slice.

Now we can access instruction results and arguments as well as EBB
arguments as slices.

Delete the Values iterator which was traversing the linked lists of
values. It is no longer needed.

lib/cretonne/src/ir/builder.rs

@@ -143,7 +143,7 @@ impl<'f> InstBuilderBase<'f> for ReplaceBuilder<'f> {
         assert_eq!(old_second_value,
                    None,
                    "Secondary result values {:?} would be left dangling by replacing {} with {}",
-                   self.dfg.inst_results(self.inst).collect::<Vec<_>>(),
+                   self.dfg.inst_results(self.inst),
                    self.dfg[self.inst].opcode(),
                    data.opcode());