Convert return formats to value lists.

With the Return and ReturnReg formats converted to using value lists for
storing their arguments, thee are no remaining instruction formats with
variable argument lists in boxed storage.

The Return and ReturnReg formats are also going to be merged since
they are identical now.

lib/cretonne/src/write.rs

@@ -282,19 +282,19 @@ fn write_instruction(w: &mut Write,
                      args[0],
                      DisplayValues(&args[1..]))
         }
-        Return { ref data, .. } => {
-            if data.varargs.is_empty() {
+        Return { ref args, .. } => {
+            if args.is_empty() {
                 writeln!(w, "")
             } else {
-                writeln!(w, " {}", data.varargs)
+                writeln!(w,
+                         " {}",
+                         DisplayValues(args.as_slice(&func.dfg.value_lists)))
             }
         }
-        ReturnReg { ref data, .. } => {
-            if data.varargs.is_empty() {
-                writeln!(w, " {}", data.arg)
-            } else {
-                writeln!(w, " {}, {}", data.arg, data.varargs)
-            }
+        ReturnReg { ref args, .. } => {
+            writeln!(w,
+                     " {}",
+                     DisplayValues(args.as_slice(&func.dfg.value_lists)))
         }
     }
 }