Have definitions retain a pointer to their parent block

kernel/modelverse_jit/cfg_ir.py

@@ -53,6 +53,7 @@ class BasicBlock(object):
             assert isinstance(value, Value) or value is None
             return Definition(
                 self.counter.next_value(),
+                self,
                 self.definition_counter.next_value(),
                 value)
 
@@ -76,8 +77,9 @@ class BasicBlock(object):
 
 class Definition(object):
     """Maps a value to a variable."""
-    def __init__(self, index, definition_index, value):
+    def __init__(self, index, block, definition_index, value):
         self.index = index
+        self.block = block
         self.definition_index = definition_index
         self.value = value
         if value is not None: