Simplify the back-end of InstBuilder.

We don't want to distinguish between single-result and multiple-result
instructions any longer.

- Merge the simple_instruction() and complex_instruction() builder
  methods into a single build() that can handle all cases.
- All format constructors now take a ctrl_type argument. Previously,
  some would take a result_type argument.
- Instruction constructors no longer attempt to compute a single result
  type. Just pass a ctrl_type and let the backend decide.

Fix one format constructor call in legalizer/split.rs which now takes a
ctrl_type instead of a result type.

lib/cretonne/src/legalizer/split.rs

@@ -228,7 +228,7 @@ fn split_value(dfg: &mut DataFlowGraph,
                 // need to insert a split instruction before returning.
                 pos.goto_top(ebb);
                 pos.next_inst();
-                let concat_inst = dfg.ins(pos).Binary(concat, ty, lo, hi).0;
+                let concat_inst = dfg.ins(pos).Binary(concat, split_type, lo, hi).0;
                 let concat_val = dfg.first_result(concat_inst);
                 dfg.change_to_alias(value, concat_val);