Some drivers (AMD Radeon RX 6700 XT, with radeonsi from Mesa 22.2.0-rc3) emit
less than one invocation per pixel, presumably because they detect that the
shader control flow is uniform for all pixels. Having the control flow depend on
SV_Position avoids this test failure.
Signed-off-by: Zebediah Figura <zfigura@codeweavers.com>