ir3: Use NIR info to enable per sample shading

NIR is tracking this better than us.

New CTS tests seem to add the requirement where in the presence of some
builtin's like gl_SampleID in a shader, even if unused, sample shading
is expected to be enabled.

See https://gitlab.khronos.org/Tracker/vk-gl-cts/-/issues/3712

Fixes:
 dEQP-VK.draw.*.implicit_sample_shading.sample_id_static_use
 dEQP-VK.draw.*.implicit_sample_shading.sample_position_static_use

Signed-off-by: Danylo Piliaiev <dpiliaiev@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24193>

src/freedreno/ir3/ir3_compiler_nir.c

@@ -2301,8 +2301,6 @@ emit_intrinsic(struct ir3_context *ctx, nir_intrinsic_instr *intr)
       dst[0] = ctx->instance_id;
       break;
    case nir_intrinsic_load_sample_id:
-      ctx->so->per_samp = true;
-      FALLTHROUGH;
    case nir_intrinsic_load_sample_id_no_per_sample:
       if (!ctx->samp_id) {
          ctx->samp_id = create_sysval_input(ctx, SYSTEM_VALUE_SAMPLE_ID, 0x1);
@@ -4297,12 +4295,6 @@ emit_instructions(struct ir3_context *ctx)
    /* some varying setup which can't be done in setup_input(): */
    if (ctx->so->type == MESA_SHADER_FRAGMENT) {
       nir_foreach_shader_in_variable (var, ctx->s) {
-         /* if any varyings have 'sample' qualifer, that triggers us
-          * to run in per-sample mode:
-          */
-         if (var->data.sample)
-            ctx->so->per_samp = true;
-
          /* set rasterflat flag for front/back color */
          if (var->data.interpolation == INTERP_MODE_NONE) {
             switch (var->data.location) {
@@ -5028,6 +5020,8 @@ ir3_compile_shader_nir(struct ir3_compiler *compiler,
        ctx->s->info.fs.post_depth_coverage)
       so->post_depth_coverage = true;
 
+   ctx->so->per_samp = ctx->s->info.fs.uses_sample_shading;
+
 out:
    if (ret) {
       if (so->ir)