pd.cut() with user-specified labels and bins arguments does not use user-provided labels
#### Code Sample

```python
import pandas as pd

bins = pd.IntervalIndex.from_tuples([(0, 2), (2, 2.5), (2.5, 5)])
pd.cut([1, 1.5, 2, 2.5, 3, 3.5], bins=bins, labels=['a', 'b', 'c']).tolist()

# output of pd.cut(...) is:
# [Interval(0.0, 2.0, closed='right'), Interval(0.0, 2.0, closed='right'), Interval(0.0, 2.0, closed='right'), Interval(2.0, 2.5, closed='right'), Interval(2.5, 5.0, closed='right'), Interval(2.5, 5.0, closed='right')]
```

#### Problem description

When labels are provided as an argument to pd.cut() with user-specified bins, then the output does not use the labels argument. 

#### Expected Output

```python
['a', 'a', 'b', 'b', 'c', 'c']
```

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.13.final.0
python-bits: 64
OS: Darwin
OS-release: 17.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: None.None

pandas: 0.23.0
pytest: 3.0.5
pip: 10.0.1
setuptools: 39.0.1
Cython: 0.25.2
numpy: 1.14.2
scipy: 1.0.1
pyarrow: None
xarray: None
IPython: 5.3.0
sphinx: None
patsy: 0.5.0
dateutil: 2.7.2
pytz: 2018.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.2.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.6.0
html5lib: 0.9999999
sqlalchemy: None
pymysql: 0.7.11.None
psycopg2: None
jinja2: 2.9.5
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
