In Trigonometry::shallowReduceInverseFunction, the reduction of
atan(1/x) used the hard value of pi instead of its equivalent in the
selected angle unit.
This fixes a bug causing atan(1/tan(40°)) to be equal to π/2-40 instead
of 50.
Change-Id: Ifd7df5cd168fadf3146eee168c0bc2136ea15a7b