Add dfg.inst_args(_mut) methods.

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

lib/cretonne/src/regalloc/coloring.rs

@@ -302,7 +302,7 @@ 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)[arg_index as usize];
+                            let arg = dfg.inst_args(inst)[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

@@ -320,7 +320,7 @@ impl Liveness {
                     .unwrap_or(&[])
                     .iter();
 
-                for &arg in func.dfg[inst].arguments(&func.dfg.value_lists) {
+                for &arg in func.dfg.inst_args(inst) {
                     // Get the live range, create it as a dead range if necessary.
                     let lr = get_or_create(&mut self.ranges, arg, func, recipe_constraints);