BUG: DateOffset applied to DatetimeIndex losing freq starting in v1.1.0
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

#### Code Sample, a copy-pastable example

```python
import pandas as pd

dtsa = pd.date_range('1998', periods = 3, freq = 'A')
s1 = pd.Series([4,5,6], index=dtsa)
s1.index
# DatetimeIndex(['1998-12-31', '1999-12-31', '2000-12-31'], dtype='datetime64[ns]', freq='A-DEC')

i1 = s1.index + pd.DateOffset(years=-1, days=1)
i1
# DatetimeIndex(['1998-01-01', '1999-01-01', '2000-01-01'], dtype='datetime64[ns]', freq=None)
```

#### Problem description

I expected **i1** to have the frequency "AS-JAN" instead of None. This behavior existed prior to version 1.1.0. For example, in 1.0.5 the output of the same commands is:
```python
> DatetimeIndex(['1998-01-01', '1999-01-01', '2000-01-01'], dtype='datetime64[ns]', freq='AS-JAN')
```

#### Expected Output

```python
> DatetimeIndex(['1998-01-01', '1999-01-01', '2000-01-01'], dtype='datetime64[ns]', freq='AS-JAN')
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : d9fff2792bf16178d4e450fe7384244e50635733
python           : 3.8.0.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.17134
machine          : AMD64
processor        : Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : English_United States.1252

pandas           : 1.1.0
numpy            : 1.19.2
pytz             : 2020.5
dateutil         : 2.8.1
pip              : 20.3.3
setuptools       : 51.0.0.post20201207
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.19.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
