Adding TimedeltaIndex to tz-aware DatetimeIndex: TypeError: data type not understood
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

dates = pd.date_range('2017-01-01', '2017-01-31', tz='UTC')
deltas = pd.TimedeltaIndex(pd.np.arange(0, len(dates)), unit='m')
dates + deltas

# Removing the tz='UTC' (making dates tz naive) works
```
throws:
```python
TypeError                                 Traceback (most recent call last)
<ipython-input-28-708df3869f57> in <module>()
      3 dates = pd.date_range('2017-01-01', '2017-01-31', tz='UTC')
      4 deltas = pd.TimedeltaIndex(pd.np.arange(0, len(dates)), unit='m')
----> 5 dates + deltas

~/miniconda/lib/python3.6/site-packages/pandas/core/indexes/datetimelike.py in __add__(self, other)
    643             from pandas.tseries.offsets import DateOffset
    644             if isinstance(other, TimedeltaIndex):
--> 645                 return self._add_delta(other)
    646             elif isinstance(self, TimedeltaIndex) and isinstance(other, Index):
    647                 if hasattr(other, '_add_delta'):

~/miniconda/lib/python3.6/site-packages/pandas/core/indexes/datetimes.py in _add_delta(self, delta)
    814             new_values = self._add_delta_td(delta)
    815         elif isinstance(delta, TimedeltaIndex):
--> 816             new_values = self._add_delta_tdi(delta)
    817             # update name when delta is Index
    818             name = com._maybe_match_name(self, delta)

~/miniconda/lib/python3.6/site-packages/pandas/core/indexes/datetimelike.py in _add_delta_tdi(self, other)
    727             mask = (self._isnan) | (other._isnan)
    728             new_values[mask] = iNaT
--> 729         return new_values.view(self.dtype)
    730 
    731     def isin(self, values):

TypeError: data type not understood
```

#### Problem description
Adding TimedeltaIndex to timezone-aware DatetimeIndex throws a TypeError.  The error goes away if the DatetimeIndex does not have a timezone.

(I vaguely remember seeing an issue on this, but can't seem to find it now when I search.)

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.10.0-33-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: C.UTF-8
LANG: C.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.21.0
pytest: None
pip: 9.0.1
setuptools: 36.5.0.post20170921
Cython: None
numpy: 1.13.3
scipy: 0.19.1
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: None
tables: 3.4.2
numexpr: 2.6.4
feather: None
matplotlib: 2.1.0
openpyxl: None
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.6.0
html5lib: 0.9999999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
