Simplify the arguments() return type.

Now that variable arguments are always stored in a value list with the
fixed arguments, we no longer need the arcane [&[Value]; 2] return type.

Arguments are always stored contiguously, so just return a &[Value]
slice.

Also remove the each_arg() methods which were just trying to make it
easier to work with the old slice pair.

lib/cretonne/src/write.rs

@@ -157,7 +157,7 @@ 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).iter().flat_map(|x| x.iter()) {
+    for &arg in func.dfg[inst].arguments(&func.dfg.value_lists) {
         let resolved = func.dfg.resolve_aliases(arg);
         if resolved != arg {
             writeln!(w, "{1:0$}{2} -> {3}", indent, "", arg, resolved)?;