Add an Index<Value> implementation to Liveness.

Use it to access live ranges that are supposed to be there.

lib/cretonne/src/regalloc/live_value_tracker.rs

@@ -260,10 +260,7 @@ impl LiveValueTracker {
         // Add the values defined by `inst`.
         let first_def = self.live.values.len();
         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),
-            };
+            let lr = &liveness[value];
             assert_eq!(lr.def(), inst.into());
             match lr.def_local_end().into() {
                 ExpandedProgramPoint::Inst(endpoint) => {