BUG: to_offset doesn't use the 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.


#### Problem description

When checking lines: https://github.com/pandas-dev/pandas/blob/master/pandas/_libs/tslibs/offsets.pyx#L3582
and https://github.com/pandas-dev/pandas/blob/master/pandas/_libs/tslibs/offsets.pyx#L3456

we can see that there is:
```python
opattern = re.compile(
    r"([+\-]?\d*|[+\-]?\d*\.\d*)\s*([A-Za-z]+([\-][\dA-Za-z\-]+)?)"
)
```
and 
```python
split = re.split(opattern, freq)
```

Which shows there's a compiled regex but it's not used.

#### Expected Output
Instead, it  should be:
```python
split = opattern.split(freq)
```

If you ever like this kind of thing...
```bash
>>> timeit.timeit("opattern.split(freq)", setup='''import re;opattern = re.compile(r"([+\-]?\d*|[+\-]?\d*\.\d*)\s*([A-Za-z]+([\-][\dA-Za-z\-]+)?)");freq="1Min"''', number=1000000)
0.6493891229999917
>>> timeit.timeit("re.split(opattern, freq)", setup='''import re;opattern = re.compile(r"([+\-]?\d*|[+\-]?\d*\.\d*)\s*([A-Za-z]+([\-][\dA-Za-z\-]+)?)");freq="1Min"''', number=1000000)
1.9217748190000066
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : 9d598a5e1eee26df95b3910e3f2934890d062caa
python           : 3.9.1.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 18.6.0
Version          : Darwin Kernel Version 18.6.0: Thu Apr 25 23:16:27 PDT 2019; root:xnu-4903.261.4~2/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.UTF-8

pandas           : 1.2.1
numpy            : 1.20.1
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 20.3.3
setuptools       : 51.1.1
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.3
IPython          : None
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
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
