Keep EBB arguments in a ValueList.

This is the first step of the value list refactoring which will replace
linked lists of values with value lists.

- Keep a ValueList in the EbbData struct containing all the EBB
  arguments.
- Change dfg.ebb_args() to return a slice instead of an iterator.

This leaves us in a temporary hybrid state where we maintain both a
linked list and a ValueList vector of the EBB arguments.

lib/cretonne/src/regalloc/live_value_tracker.rs

@@ -182,7 +182,7 @@ impl LiveValueTracker {
 
         // Now add all the live arguments to `ebb`.
         let first_arg = self.live.values.len();
-        for value in dfg.ebb_args(ebb) {
+        for &value in dfg.ebb_args(ebb) {
             let lr = liveness
                 .get(value)
                 .expect("EBB argument value has no live range");