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/ir/dfg.rs

@@ -633,7 +633,12 @@ impl DataFlowGraph {
             .expect("Instruction has no results")
     }
 
-    /// Iterate through all the results of an instruction.
+    /// Test if `inst` has any result values currently.
+    pub fn has_results(&self, inst: Inst) -> bool {
+        !self.results[inst].is_empty()
+    }
+
+    /// Return all the results of an instruction.
     pub fn inst_results(&self, inst: Inst) -> &[Value] {
         self.results[inst].as_slice(&self.value_lists)
     }