nir/lower_point_size_mov: handle case where gl_Position isn't written

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15821>

src/compiler/nir/nir_lower_point_size_mov.c

@@ -65,6 +65,7 @@ lower_impl(nir_function_impl *impl,
       b.cursor = nir_before_cf_list(&impl->body);
       nir_copy_var(&b, new_out, in);
    } else {
+      bool found = false;
       nir_foreach_block_safe(block, impl) {
          nir_foreach_instr_safe(instr, block) {
             if (instr->type == nir_instr_type_intrinsic) {
@@ -74,11 +75,16 @@ lower_impl(nir_function_impl *impl,
                   if (var == out) {
                      b.cursor = nir_after_instr(instr);
                      nir_copy_var(&b, new_out ? new_out : out, in);
+                     found = true;
                   }
                }
             }
          }
       }
+      if (!found) {
+         b.cursor = nir_before_cf_list(&impl->body);
+         nir_copy_var(&b, new_out, in);
+      }
    }
 
    nir_metadata_preserve(impl, nir_metadata_block_index |