ENH: allow passing a mask to nanops to facilitate EA reduction ops
https://github.com/pandas-dev/pandas/pull/22762#discussion_r218832793

currently we call nanops like

``nansum(values, skipna=boolean)``

this doesn't allow us to take advantage of knowing the nan mask a-priori (e.g. in an IntegerArray), and instead re-computes it based on the values (only). 

We could allow ``skipna=`` to take a mask as well here (or add a ``mask=`` keyword) to facilitate this.