nir/lower_pntc_ytransform: Support PointCoordIsSysval

nir/lower_pntc_ytransform: Support PointCoordIsSysval

Pattern match the point coord sysval and support lowering it as well.
This is required to handle flipped framebuffers on Bifrost. However,
what this pass normalizes to is the opposite of the hardware mode we
used on Bifrost before, so we need to swap modes at the same time to
prevent regressions.

Fixes Piglit glsl-fs-pointcoord and glsl-fs-pointcoord_gles2

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13073>