i965/fs: Add a comment about about an implementation detail.

I was going to fix the code above like the previous commit, but we already
had that covered (otherwise all our uniform access would have been broken,
unlike just pull constants).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/i965/brw_fs_reg_allocate.cpp

@@ -310,6 +310,10 @@ fs_visitor::setup_payload_interference(struct ra_graph *g,
        * node.
        */
       for (int j = 0; j < this->virtual_grf_count; j++) {
+         /* Note that we use a <= comparison, unlike virtual_grf_interferes(),
+          * in order to not have to worry about the uniform issue described in
+          * calculate_live_intervals().
+          */
          if (this->virtual_grf_def[j] <= payload_last_use_ip[i] ||
              this->virtual_grf_use[j] <= payload_last_use_ip[i]) {
             ra_add_node_interference(g, first_payload_node + i, j);