Add dfg.inst_args(_mut) methods.

A shortcut for calling arguments() directly that goes with the existing
inst_results() method.

lib/cretonne/src/write.rs

@@ -160,7 +160,7 @@ fn type_suffix(func: &Function, inst: Inst) -> Option<Type> {
 
 // Write out any value aliases appearing in `inst`.
 fn write_value_aliases(w: &mut Write, func: &Function, inst: Inst, indent: usize) -> Result {
-    for &arg in func.dfg[inst].arguments(&func.dfg.value_lists) {
+    for &arg in func.dfg.inst_args(inst) {
         let resolved = func.dfg.resolve_aliases(arg);
         if resolved != arg {
             writeln!(w, "{1:0$}{2} -> {3}", indent, "", arg, resolved)?;