fix: use is None instead of == None in test_tir_constructor.py

Author: harshadkhetpal

tests/python/tirx-base/test_tir_constructor.py

@@ -29,7 +29,7 @@ def test_expr_constructor():
 
     x = tvm.tirx.Reduce(None, [1], [tvm.tirx.IterVar((0, 1), "x", 2)], None, 0)
     assert isinstance(x, tvm.tirx.Reduce)
-    assert x.combiner == None
+    assert x.combiner is None
     assert x.value_index == 0
 
     x = tvm.tirx.FloatImm("float32", 1.0)