spirv: make gl_HelperInvocation volatile if demote is being used

Non-volatile gl_HelperInvocation after demote is undefined.
In order to avoid application bugs, make it volatile if we use demote.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27617>

src/compiler/spirv/vtn_variables.c

@@ -1600,6 +1600,15 @@ var_decoration_cb(struct vtn_builder *b, struct vtn_value *val, int member,
    case SpvDecorationCounterBuffer:
       /* Counter buffer decorations can safely be ignored by the driver. */
       return;
+   case SpvDecorationBuiltIn:
+      /* Non-volatile gl_HelperInvocation after demote is undefined.
+       * In order to avoid application bugs, make it volatile if we use demote.
+       */
+      if (dec->operands[0] == SpvBuiltInHelperInvocation &&
+          (b->enabled_capabilities.DemoteToHelperInvocation ||
+           b->convert_discard_to_demote))
+         vtn_var->access |= ACCESS_VOLATILE;
+      break;
    default:
       break;
    }