message: [TF:MLIR] Add TFExecutorGraphPruning pass to TFStandardPipeline.
  This is to remove dead islands before running TFExecutorIslandCoarsening pass.
diff: @@ -50,10 +50,12 @@ void CreateTFStandardPipeline(OpPassManager &pm,
  OpPassManager &func_pm = pm.nest<FuncOp>();
  // First operates on the executor dialect:
  - // - eliminate trivial switch/merge
  + // - eliminate trivial switch/merge.
  + // - remove dead islands.
  // - fuse islands as much as possible.
  // - materialize the eventual "pass-through" ops by inlining their content.
  func_pm.addPass(tf_executor::CreateSwitchFoldPass());
  + func_pm.addPass(tf_executor::CreateTFExecutorGraphPruningPass());
  func_pm.addPass(tf_executor::CreateTFExecutorIslandCoarseningPass());
  func_pm.addPass(CreateMaterializePassthroughOpPass());
  