Add Apply.inst_predicate().

Compute an instruction predicate from any constant values given as
arguments for the immediate operands in an instruction pattern.

Allows for patterns like icmp.i32(intcc.ugt, x, y) or iadd_imm.i32(x, 1)

Trap these predicates in the legalizer code generator since we can't
actually handle them yet.

lib/cretonne/meta/cdsl/predicates.py

@@ -216,7 +216,8 @@ class FieldPredicate(object):
         This predicate can be evaluated in the context of an instruction
         format.
         """
-        return self.field.format
+        iform = self.field.format  # type: InstructionFormat
+        return iform
 
     def predicate_leafs(self, leafs):
         # type: (Set[PredLeaf]) -> None