datetime comparison from pandas.core.seroes.Series to numpy.datetime64 fail for pandas 0.21
#### Code Sample, a copy-pastable example if possible

pandas 0.20.3
![image](https://user-images.githubusercontent.com/4520383/33405027-a5151e48-d534-11e7-8052-d64a2948c9f5.png)


```python
import pandas as pd
rng = pd.DataFrame({ "A" : pd.date_range('1/1/2011', periods=720, freq='H')})
myvar = rng["A"] < rng["A"].values[3]
```
Works.
![image](https://user-images.githubusercontent.com/4520383/33404978-76cbb9ca-d534-11e7-8310-ca0248de483c.png)


pandas 0.21
![image](https://user-images.githubusercontent.com/4520383/33404865-e69641f4-d533-11e7-900e-c0974934f845.png)
exact same code:
![image](https://user-images.githubusercontent.com/4520383/33404901-04733434-d534-11e7-937e-287d33d2e248.png)




I am not sure why this would fail now?
Note: it might be fixed in #18188