BUG: Index of RHS of series assignment ignored
- [ 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.

---

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

```python
>>> x = pd.Series([1, 2, 3])
>>> y = pd.Series([4, 5, 6], index=[2, 1, 0])
>>> x.iloc[:] = y
>>> x
0    4
1    5
2    6
dtype: int64
```

#### Problem description
When assigning a series to another, the index of the RHS should be considered. In this example, the mapping of the RHS is {2: 4, 1: 5, 0: 6}, but such mapping is ignored in the final result.

#### Expected Output
```
>>> x
0    6
1    5
2    4
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : 3e89b4c4b1580aa890023fc550774e63d499da25
python           : 3.7.8.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.15.0-76-generic
Version          : #86-Ubuntu SMP Fri Jan 17 17:24:28 UTC 2020
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : en_US.UTF-8

pandas           : 1.2.0
numpy            : 1.19.1
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.2.3
setuptools       : 49.6.0.post20200814
Cython           : 0.29.21
pytest           : 6.0.2
hypothesis       : 5.28.0
sphinx           : 3.2.1
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.3.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : 0.8.2
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.1
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 1.0.1
pyxlsb           : None
s3fs             : None
scipy            : 1.5.2
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : 0.16.2
xlrd             : None
xlwt             : None
numba            : 0.52.0
</details>
