REGR: pandas 1.2rc fails merge with AttributeError: 'bool' object has no attribute 'all'
From the dask test suite:

```
left = pd.DataFrame({"x": [1, 1], "z": ["foo", "foo"]})
right = pd.DataFrame({"x": [1, 1], "z": ["foo", "foo"]})

pd.merge(left, right, how="right", left_index=True, right_on="x")
```

This fails on master / 1.2.0rc:

```
In [18]: pd.merge(left, right, how="right", left_index=True, right_on="x")
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-18-94bf9fb575b1> in <module>
      2 right = pd.DataFrame({"x": [1, 1], "z": ["foo", "foo"]})
      3 
----> 4 pd.merge(left, right, how="right", left_index=True, right_on="x")

~/scipy/pandas/pandas/core/reshape/merge.py in merge(left, right, how, on, left_on, right_on, left_index, right_index, sort, suffixes, copy, indicator, validate)
     93         validate=validate,
     94     )
---> 95     return op.get_result()
     96 
     97 

~/scipy/pandas/pandas/core/reshape/merge.py in get_result(self)
    710             result = self._indicator_post_merge(result)
    711 
--> 712         self._maybe_add_join_keys(result, left_indexer, right_indexer)
    713 
    714         self._maybe_restore_index_levels(result)

~/scipy/pandas/pandas/core/reshape/merge.py in _maybe_add_join_keys(self, result, left_indexer, right_indexer)
    866                 if mask_left.all():
    867                     key_col = rvals
--> 868                 elif mask_right.all():
    869                     key_col = lvals
    870                 else:

AttributeError: 'bool' object has no attribute 'all'

```