PERF: avoid extra copies in ._where

[jbrockmendel]

In NDFrame._where we do

         # align the cond to same shape as myself
         cond = common.apply_if_callable(cond, self)
         if isinstance(cond, NDFrame):
            # CoW: Make sure reference is not kept alive
            cond = cond.align(self, join="right", broadcast_axis=1, copy=False)[0]

        [...]
         # make sure we are boolean
        fill_value = bool(inplace)
        cond = cond.fillna(fill_value)

        [...]
        cond = -cond if inplace else cond
        cond = cond.reindex(self._info_axis, axis=self._info_axis_number, copy=False)

I think we can avoid making some copies of cond by

a) if we do an align, pass fill_value=bool(inplace) there to avoid having to fill it later.
b) check if we need to do fillna before doing it (maybe that check itself is expensive? may be irrelevant with CoW?)
c) maybe avoid doing the cond = -cond if ... by handling that at a lower level?

These are all pretty speculative, salvaged from an old branch that was collecting dust. Might be a good 4th issue.

[phofl]

a) sounds sensible
b) should be irrelevant with CoW, yes.