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/regalloc/live_value_tracker.rs

@@ -233,7 +233,7 @@ impl LiveValueTracker {
 
         // Add the values defined by `inst`.
         let first_def = self.live.values.len();
-        for value in dfg.inst_results(inst) {
+        for &value in dfg.inst_results(inst) {
             let lr = match liveness.get(value) {
                 Some(lr) => lr,
                 None => panic!("{} result {} has no live range", dfg[inst].opcode(), value),