Implement __str__, __repr__ for tree_ir.VariableName

kernel/modelverse_jit/tree_ir.py

@@ -802,6 +802,12 @@ class VariableName(object):
            instruction if it is not None."""
         return self.name
 
+    def __str__(self):
+        return self.name
+
+    def __repr__(self):
+        return 'VariableName(%r)' % self.name
+
 class VariableInstruction(Instruction):
     """A base class for instructions that access variables."""
     def __init__(self, name):