pan/bi: Don't read alpha out of bounds

RT#0 may not have alpha, do something safe instead of risking issues
with RA later.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9105>

src/panfrost/bifrost/bifrost_compile.c

@@ -441,6 +441,10 @@ bi_emit_fragment_out(bi_builder *b, nir_intrinsic_instr *instr)
                         (T == nir_type_float32) ? bi_word(rgba, 3) :
                         bi_dontcare();
 
+                /* Don't read out-of-bounds */
+                if (nir_src_num_components(instr->src[0]) < 4)
+                        alpha = bi_imm_f32(1.0);
+
                 bi_instr *atest = bi_atest_to(b, bi_register(60),
                                 bi_register(60), alpha);
                 b->shader->emitted_atest = true;