mesa/arbprog: Move the GLSLFragCoordIsSysVal handling to prog_to_nir.

mesa/arbprog: Move the GLSLFragCoordIsSysVal handling to prog_to_nir.

We don't need to go grubbing around in the ARB program when we can use the
right variable type at prog_to_nir time.  This does leave
fp->system_values_read/inputs_read as they were, but I don't see anywhere
that that matters (the NIR will have its info gathered appropriately, and
other lowering may also cause mismatch between the gl_program and the
NIR).

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17528>