DatetimeIndex + Series[timedelta64] returns DatetimeIndex
This is listed in the roundup #18824 but the behavior has changed since the older issue.

```
dti = pd.date_range('2017-01-01', periods=2)
ser = pd.Series(pd.TimedeltaIndex(['1 Day', '1Day']))
res1 = dti + ser
res2 = ser + dti
res3 = dti - ser
res4 = -ser + dti

>>> res1
DatetimeIndex(['2017-01-02', '2017-01-03'], dtype='datetime64[ns]', freq=None)
>>> res2
0   2017-01-02
1   2017-01-03
dtype: datetime64[ns]
>>> res3
DatetimeIndex(['2016-12-31', '2017-01-01'], dtype='datetime64[ns]', freq=None)
>>> res4
0   2016-12-31
1   2017-01-01
dtype: datetime64[ns]
```