BUG: Different results from Series.replace with compiled regex
- [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.

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

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

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

```python
# Your code here
import re
import pandas as pd

print(pd.__version__)

s = pd.Series(['a', 'aa', 'b'])
regex = {'^a$': 1, '^b$': 2, '^aa$': 3}
print(s.replace(regex, regex=True)) 
print(s.replace({re.compile(k): v for k,v in regex.items()}, regex=True))


```

#### Problem description
These two `.replace` calls are equivalent on pandas `1.0.5`. They are different on `1.1.0` and `master`.

```
1.2.0.dev0+79.g3c87b019b
0    1
1    3
2    2
dtype: int64
0     a
1    aa
2     b
dtype: object


1.0.5
0    1
1    3
2    2
dtype: int64
0    1
1    3
2    2
dtype: int64
```

#### Expected Output


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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.7.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 63 Stepping 2, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 1.0.5
numpy            : 1.19.1
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.2.1
setuptools       : 49.3.1.post20200810
Cython           : 0.29.21
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None

</details>
