Move the ctrl_typevar function into dfg.

Soon, InstructionData won't have sufficient information to compute this.

Give TargetIsa::encode() an explicit ctrl_typevar argument. This
function does not require the instruction to be inserted in the DFG
tables.

lib/cretonne/src/write.rs

@@ -152,7 +152,7 @@ fn type_suffix(func: &Function, inst: Inst) -> Option<Type> {
         }
     }
 
-    let rtype = inst_data.ctrl_typevar(&func.dfg);
+    let rtype = func.dfg.ctrl_typevar(inst);
     assert!(!rtype.is_void(),
             "Polymorphic instruction must produce a result");
     Some(rtype)