message: Disable using MLIR for VarIsInitializedOp via XlaCompilationCache::CompileSingleOp.
  TPU on demand mode can trigger for VarIsInitializedOp, which is not supported yet via MLIR for compilation.
diff: @@ -289,8 +289,10 @@ Status XlaCompilationCache::CompileSingleOp(
  return arg.kind == XlaCompiler::Argument::kTensorList;
  });
  const ConfigProto* config = ctx->function_library()->config_proto();
  + // TODO(b/171039585): Support tf.VarIsInitializedOp using MLIR.
  bool use_mlir = config && config->experimental().enable_mlir_bridge() &&
  - !has_tensor_list_arg;
  + !has_tensor_list_arg &&
  + node_def.op() != "VarIsInitializedOp";
  #ifdef LIBTPU_ON_GCE
  if (use_mlir) {
  LOG(WARNING) << "MLIR is not supported in this environment.";
  