nir/lower_blend: No-op nir_color_mask if no mask

In this usual case, do a quick check to avoid generating 5 useless instructions
(mov/vec4 instructions). They'll get copypropped but that creates more work for
the optimizer and nir/lower_blend runs in a hot variant path on both Asahi and
Panfrost.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Acked-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20016>

src/compiler/nir/nir_lower_blend.c

@@ -503,8 +503,9 @@ nir_lower_blend_store(nir_builder *b, nir_intrinsic_instr *store,
       blended = nir_blend(b, options, rt, src, options->src1, dst);
    }
 
-   /* Apply a colormask */
-   blended = nir_color_mask(b, options->rt[rt].colormask, blended, dst);
+   /* Apply a colormask if necessary */
+   if (options->rt[rt].colormask != BITFIELD_MASK(4))
+      blended = nir_color_mask(b, options->rt[rt].colormask, blended, dst);
 
    const unsigned num_components = glsl_get_vector_elements(var->type);