ir3: Detect FS that write only color without other side effects

Signed-off-by: Danylo Piliaiev <dpiliaiev@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33735>

src/freedreno/ir3/ir3_compiler_nir.c

@@ -6043,6 +6043,9 @@ ir3_compile_shader_nir(struct ir3_compiler *compiler,
 
    if (so->type == MESA_SHADER_FRAGMENT) {
       so->empty = is_empty(ir) && so->num_sampler_prefetch == 0;
+      so->writes_only_color = !ctx->s->info.writes_memory && !so->has_kill &&
+                              !so->writes_pos && !so->writes_smask &&
+                              !so->writes_stencilref;
    }
 
    if (gl_shader_stage_is_compute(so->type)) {

src/freedreno/ir3/ir3_shader.h

@@ -854,6 +854,8 @@ struct ir3_shader_variant {
    bool post_depth_coverage;
 
    bool empty;
+   /* Doesn't have side-effects, no kill, no D/S write, etc. */
+   bool writes_only_color;
 
    /* Are we using split or merged register file? */
    bool mergedregs;