[BugFix][TVMScript] Parser crash

[lightzhan-intellif]

This PR tries to fix the crash of parser when the old value of a var is an array but the new value is not. For example:

from tvm.script import tir as T
def func_wrapper(shape, dtype):
    @T.prim_func
    def test_case():
        a = T.alloc_buffer(shape, dtype=dtype)
    
    return test_case


if __name__ == "__main__":
    a = np.zeros((10, 10), dtype="int8")
    print(func_wrapper((256, 256), dtype="int8").script())

In the above code, there are two assignment to var 'a'. In the global scope, its value is a numpy array. But it is a Buffer in the prim function. There is a table named 'name2value' to track the value of vars like 'a' here.
When the parser wants to update its value, it will compare the value between the new and the old assignment. Here the problem comes. When we use '==' to compare an array with a value, the result is an array too, which can not be used as a condition of a if stmt directly. So, the code above will emit an error:

error: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()
 --> /workspace/code_newest/tvm/private_test/test_meta_programming.py:16:9
    |  
 16 |          a = T.alloc_buffer(shape, dtype=dtype)
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This PR fixes this by change "==" to "is".

[tvm-bot]

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