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/write.rs

@@ -95,7 +95,7 @@ pub fn write_ebb_header(w: &mut Write, func: &Function, ebb: Ebb) -> Result {
         write!(w, "                    ")?;
     }
 
-    let mut args = func.dfg.ebb_args(ebb);
+    let mut args = func.dfg.ebb_args(ebb).iter().cloned();
     match args.next() {
         None => return writeln!(w, "{}:", ebb),
         Some(arg) => {