Little change to Function.__new__ logic.
Little change to Function.__new__ logic.

After the previous patches we have that every Function class has nargs defined
but undefined Functions, so we can skip hasattr(cls, 'undefined_Function')
check.

The if is moved up to optimize the checks a little more.

Signed-off-by: Riccardo Gori <01dbbf934a86d79f3d80b4b5a86f5600765ffcc2@gmail.com>
Signed-off-by: Ondrej Certik <b816faa87b7d35274d2e545c5be11ed4376f3ccf@certik.cz>

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