Compute the controlling type variable accurately.

Some polymorphic instructions don't return the controlling type
variable, so it has to be computed from the designated operand instead.

- Add a requires_typevar_operand() method to the operand constraints
  which indicates that.
- Add a ctrl_typevar(dfg) method to InstructionData which computes the
  controlling type variable correctly, and returns VOID for monomorphic
  instructions.
- Use ctrl_typevar(dfg) to drive the level-1 encoding table lookups.

lib/cretonne/src/isa/intel/mod.rs

@@ -53,8 +53,8 @@ impl TargetIsa for Isa {
         registers::INFO.clone()
     }
 
-    fn encode(&self, _: &DataFlowGraph, inst: &InstructionData) -> Result<Encoding, Legalize> {
-        lookup_enclist(inst.first_type(),
+    fn encode(&self, dfg: &DataFlowGraph, inst: &InstructionData) -> Result<Encoding, Legalize> {
+        lookup_enclist(inst.ctrl_typevar(dfg),
                        inst.opcode(),
                        self.cpumode,
                        &enc_tables::LEVEL2[..])