nir: Update shader info when adding discards

vc4 is about to start using the shader info field to set up discard
handling.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/nir/nir_lower_clip.c

@@ -286,6 +286,8 @@ lower_clip_fs(nir_function_impl *impl, unsigned ucp_enables,
                                               nir_intrinsic_discard_if);
          discard->src[0] = nir_src_for_ssa(cond);
          nir_builder_instr_insert(&b, &discard->instr);
+
+         b.shader->info.fs.uses_discard = true;
       }
    }
 }