Classify Vars in patterns.

There's 4 classes of variables, depending on whether they have defs in
the source and destination patterns.

Add more XForm verification: In a legalize XForm, all source defs must
be outputs.

Fix a legalize pattern bug caught by this.

lib/cretonne/meta/gen_legalizer.py

@@ -14,7 +14,7 @@ from cretonne.ast import Def, Apply  # noqa
 from cretonne.xform import XForm, XFormGroup  # noqa
 
 try:
-    from typing import Union  # noqa
+    from typing import Union
     DefApply = Union[Def, Apply]
 except ImportError:
     pass