Add a predicate_leafs() method.

This collects all of the leaf predicates that go into a compound predicate.
Current leaf predicates are:

- Settings for ISA predicates, and
- FieldPredicates for instruction predicates.

meta/cretonne/__init__.py

@@ -41,6 +41,9 @@ class Setting(object):
         """
         return self.group
 
+    def predicate_leafs(self, leafs):
+        leafs.add(self)
+
 
 class BoolSetting(Setting):
     """

meta/cretonne/predicates.py

@@ -68,6 +68,13 @@ class Predicate(object):
     def predicate_context(self):
         return self.context
 
+    def predicate_leafs(self, leafs):
+        """
+        Collect all leaf predicates into the `leafs` set.
+        """
+        for part in self.parts:
+            part.predicate_leafs(leafs)
+
 
 class And(Predicate):
     """
@@ -164,6 +171,9 @@ class FieldPredicate(object):
         """
         return self.field.format
 
+    def predicate_leafs(self, leafs):
+        leafs.add(self)
+
     def rust_predicate(self, prec):
         """
         Return a string of Rust code that evaluates this predicate.