BUG: Index._searchsorted_monotonic(..., side='right') returns the left side position for monotonic decreasing indexes
#### Code Sample, a copy-pastable example if possible

```python
In [1]: import pandas as pd
   ...: idx = pd.Index([3, 2, 1])
   ...: idx
   ...:
Out[1]: Int64Index([3, 2, 1], dtype='int64')

In [2]: idx._searchsorted_monotonic(3, side='right')
Out[2]: 0

In [3]: idx._searchsorted_monotonic(2, side='right')
Out[3]: 1

In [4]: idx._searchsorted_monotonic(1, side='right')
Out[4]: 2
```
#### Problem description
The left side positions are returned even though `side='right'` is specified.

Note that this is also the root cause of #16417

#### Expected Output
I'd expect the output to be 1, 2, 3 instead of 0, 1, 2.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.2.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 78 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.20.3
pytest: 3.1.2
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.26
numpy: 1.13.1
scipy: 0.19.1
xarray: None
IPython: 6.1.0
sphinx: 1.5.6
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.2.2
numexpr: 2.6.2
feather: None
matplotlib: 2.0.2
openpyxl: 2.4.7
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.7.3
bs4: 4.6.0
html5lib: 0.999
sqlalchemy: 1.1.9
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None

</details>
