radv: fix unused non-xfb shader outputs not being removed

It was not taken into account that without Offset decoration
the output is not written into XFB.

Aside from eliminating more outputs this change prevents gl_PerVertex
builtins generated by glslang from being kept alive in case when XFB
is enabled. Keeping such outputs alive may upset a driver.

VUID-StandaloneSpirv-Offset-04716:
    "Only variables or block members in the output interface decorated
    with Offset can be captured for transform feedback, and those
    variables or block members must also be decorated with XfbBuffer
    and XfbStride, or inherit XfbBuffer and XfbStride decorations from
    a block containing them"

Additional info about glslang behavior could be found at:
 https://github.com/KhronosGroup/glslang/issues/1526

Fixes: e95531e101
("radv: fix gathering XFB info if there is dead outputs")

Signed-off-by: Danylo Piliaiev <dpiliaiev@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24318>

src/amd/vulkan/radv_shader.c

@@ -322,7 +322,45 @@ is_not_xfb_output(nir_variable *var, void *data)
    if (var->data.mode != nir_var_shader_out)
       return true;
 
-   return !var->data.explicit_xfb_buffer && !var->data.explicit_xfb_stride;
+   /* From the Vulkan 1.3.259 spec:
+    *
+    *    VUID-StandaloneSpirv-Offset-04716
+    *
+    *    "Only variables or block members in the output interface decorated
+    *    with Offset can be captured for transform feedback, and those
+    *    variables or block members must also be decorated with XfbBuffer
+    *    and XfbStride, or inherit XfbBuffer and XfbStride decorations from
+    *    a block containing them"
+    *
+    * glslang generates gl_PerVertex builtins when they are not declared,
+    * enabled XFB should not prevent them from being DCE'd.
+    *
+    * The logic should match nir_gather_xfb_info_with_varyings
+    */
+
+   if (!var->data.explicit_xfb_buffer)
+      return true;
+
+   bool is_array_block = var->interface_type != NULL &&
+      glsl_type_is_array(var->type) &&
+      glsl_without_array(var->type) == var->interface_type;
+
+   if (!is_array_block) {
+      return !var->data.explicit_offset;
+   } else {
+      /* For array of blocks we have to check each element */
+      unsigned aoa_size = glsl_get_aoa_size(var->type);
+      const struct glsl_type *itype = var->interface_type;
+      unsigned nfields = glsl_get_length(itype);
+      for (unsigned b = 0; b < aoa_size; b++) {
+         for (unsigned f = 0; f < nfields; f++) {
+            if (glsl_get_struct_field_offset(itype, f) >= 0)
+               return false;
+         }
+      }
+
+      return true;
+   }
 }
 
 /* If the shader doesn't have an index=1 output, then assume that it meant for a location=1 to be used. This works on