intel/compiler: Clarify why VUE is recomputed by FS

FS will get the last geometry VUE, but it still needs to recompute in
case the number of position slots assigned by geometry is larger than
one -- this happens when Primitive Replication is used.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10653>

src/intel/compiler/brw_fs.cpp

@@ -1875,6 +1875,11 @@ calculate_urb_setup(const struct intel_device_info *devinfo,
           * in an order that matches the output of the previous pipeline stage
           * (geometry or vertex shader).
           */
+
+         /* Re-compute the VUE map here in the case that the one coming from
+          * geometry has more than one position slot (used for Primitive
+          * Replication).
+          */
          struct brw_vue_map prev_stage_vue_map;
          brw_compute_vue_map(devinfo, &prev_stage_vue_map,
                              key->input_slots_valid,