comparison between two PeriodIndexes doesn't validate length
```python
In [5]: idx = pd.period_range('2017', periods=12, freq="A-DEC")

In [6]: idx <= idx[[0]]
Out[6]:
array([ True, False, False, False, False, False, False, False, False,
       False, False, False])

```

This should raise.

Both `DatetimeIndex` and `TimedeltaIndex` do raise

```python
In [8]: idx <= idx[[0]]
```

```pytb
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-8-8a2a887e6efd> in <module>
----> 1 idx <= idx[[0]]

~/sandbox/pandas/pandas/core/indexes/datetimes.py in wrapper(self, other)
     90
     91     def wrapper(self, other):
---> 92         result = getattr(DatetimeArrayMixin, opname)(self, other)
     93         if is_bool_dtype(result):
     94             return result

~/sandbox/pandas/pandas/core/arrays/datetimes.py in wrapper(self, other)
    133             else:
    134                 self._assert_tzawareness_compat(other)
--> 135                 result = meth(self, np.asarray(other))
    136
    137             result = com.values_from_object(result)

~/sandbox/pandas/pandas/core/arrays/datetimelike.py in cmp_method(self, other)
     51         if isinstance(other, (np.ndarray, ABCIndexClass, ABCSeries)):
     52             if other.ndim > 0 and len(self) != len(other):
---> 53                 raise ValueError('Lengths must match to compare')
     54
     55         if needs_i8_conversion(self) and needs_i8_conversion(other):

ValueError: Lengths must match to compare

```