PERF: special case numpy.dtype in is_extension_array_dtype
Master:

```
dtype = np.dtype(float)
dtype2 = pd.Int32Dtype()

In [2]: %timeit pd.api.types.is_extension_array_dtype(dtype)
645 ns ± 59.1 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

In [3]: %timeit pd.api.types.is_extension_array_dtype(dtype2)
251 ns ± 9.68 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)
```

with this branch, it keeps the speed for ExtensionDtype the same, and improves for numpy.dtype (avoiding a extension dtype registry lookup):

```
In [2]: %timeit pd.api.types.is_extension_array_dtype(dtype)
328 ns ± 24.3 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

In [2]:  %timeit pd.api.types.is_extension_array_dtype(dtype2)
240 ns ± 19.4 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)
```