Create FormatField attributes on demand.

The InstructionFormat objects make their non-value operands available as
FormatField attributes for use by predicates etc.

Compute these on demand instead of up front. This makes it possible for
the mypy tool to infer the types of these attributes from the
__getattr__ signature.

lib/cretonne/meta/cretonne/__init__.py

@@ -707,11 +707,24 @@ class InstructionFormat(object):
             self.members.append(member)
             yield k
 
-            # Create `FormatField` instances for the immediates.
+    def __getattr__(self, attr):
-            if isinstance(k, ImmediateKind):
+        # type: (str) -> FormatField
-                assert not hasattr(self, member), "Duplicate member name"
+        """
-                field = FormatField(self, i, member)
+        Make instruction format members available as attributes.
-                setattr(self, member, field)
+
+        Each non-value format member becomes a corresponding `FormatField`
+        attribute.
+        """
+        try:
+            i = self.members.index(attr)
+        except ValueError:
+            raise AttributeError(
+                    '{} is neither a {} member or a '
+                    .format(attr, self.name) +
+                    'normal InstructionFormat attribute')
+        field = FormatField(self, i, attr)
+        setattr(self, attr, field)
+        return field
 
     @staticmethod
     def lookup(ins, outs):