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/liveness.rs

@@ -308,7 +308,7 @@ impl Liveness {
                 // Make sure we have created live ranges for dead defs.
                 // TODO: When we implement DCE, we can use the absence of a live range to indicate
                 // an unused value.
-                for def in func.dfg.inst_results(inst) {
+                for &def in func.dfg.inst_results(inst) {
                     get_or_create(&mut self.ranges, def, func, &enc_info);
                 }