BUG: PeriodIndex._shallow_copy doesn't roundtrip asi8 values correctly
On master:
```python
In [1]: import pandas as pd; pd.__version__
Out[1]: '0.24.0.dev0+1340.g8c58817bd'

In [2]: pi = pd.period_range('2018-01-01', periods=3, freq='2D')

In [3]: pi
Out[3]: PeriodIndex(['2018-01-01', '2018-01-03', '2018-01-05'], dtype='period[2D]', freq='2D')

In [4]: pi._shallow_copy(pi.asi8, freq=pi.freq)
Out[4]: PeriodIndex(['2018-01-02', '2018-01-04', '2018-01-06'], dtype='period[2D]', freq='2D')
```
It appears that values are shifted forward by `freq.n - 1` base frequencies, e.g. if `freq='5D'` then the above will result in a shift of `'2018-01-01'` --> `'2018-01-05'`.

Note that this is working on 0.23.4:
```python
In [1]: import pandas as pd; pd.__version__
Out[1]: '0.23.4'

In [2]: pi = pd.period_range('2018-01-01', periods=3, freq='2D')

In [3]: pi
Out[3]: PeriodIndex(['2018-01-01', '2018-01-03', '2018-01-05'], dtype='period[2D]', freq='2D')

In [4]: pi._shallow_copy(pi.asi8, freq=pi.freq)
Out[4]: PeriodIndex(['2018-01-01', '2018-01-03', '2018-01-05'], dtype='period[2D]', freq='2D')
```
cc @jbrockmendel : Am I correct in expecting that `Out[3]` and `Out[4]` should be the same?
