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/regalloc/coloring.rs

@@ -299,7 +299,7 @@ impl<'a> Context<'a> {
                         }
                         ConstraintKind::Tied(arg_index) => {
                             // This def must use the same register as a fixed instruction argument.
-                            let arg = dfg[inst].arguments(&dfg.value_lists)[0][arg_index as usize];
+                            let arg = dfg[inst].arguments(&dfg.value_lists)[arg_index as usize];
                             let loc = locations[arg];
                             *locations.ensure(lv.value) = loc;
                             // Mark the reused register. It's not really clear if we support tied

lib/cretonne/src/regalloc/liveness.rs

@@ -318,7 +318,7 @@ impl Liveness {
                 let mut operand_constraints =
                     recipe_constraints.get(recipe).map(|c| c.ins).unwrap_or(&[]).iter();
 
-                func.dfg[inst].each_arg(&func.dfg.value_lists, |arg| {
+                for &arg in func.dfg[inst].arguments(&func.dfg.value_lists) {
                     // Get the live range, create it as a dead range if necessary.
                     let lr = get_or_create(&mut self.ranges, arg, func, recipe_constraints);
 
@@ -333,7 +333,7 @@ impl Liveness {
                     if let Some(constraint) = operand_constraints.next() {
                         lr.affinity.merge(constraint, &reg_info);
                     }
-                });
+                }
             }
         }
     }