Reduce[z == ArcTan[1/x] + ArcTan[x], {x}, Reals]
gets you most of the way there.
This seems to do the trick:
PiecewiseExpand@Piecewise[
Join @@
With[{assum = And @@ Cases[#, Except[_Equal]]},
({z, assum} /. Solve[FullSimplify[#, assum]])] & /@
List @@ LogicalExpand@
Reduce[z == ArcTan[1/x] + ArcTan[x], {x}, Reals],
Indeterminate] (* <-- or Undefined *)