message: [XLA] Add comments for the Reduce->Reshape simplifier pass.
  Also forcing reduction order for init to be on lhs for ReduceWindow->Map pass.
diff: @@ -1774,6 +1774,10 @@ Status AlgebraicSimplifierVisitor::HandleReduce(HloInstruction* reduce) {
  new_reduce_dimensions, function));
  }
  + // If the reduction results in the same number of elements, then the only
  + // possible side effect would be a reshape. Since the init_value is an
  + // identity of the reduction function, we can therefore replace the reduce
  + // with a simple reshape, ignoring the reduction function completely.
  if (ShapeUtil::ElementsIn(reduce->shape()) ==
  ShapeUtil::ElementsIn(arg->shape())) {
  return ReplaceWithNewInstruction(
  @@ -1842,7 +1846,7 @@ Status AlgebraicSimplifierVisitor::HandleReduceWindow(
  return ReplaceWithNewInstruction(
  reduce_window,
  HloInstruction::CreateMap(reduce_window->shape(),
  - {operand, reduce_window->mutable_operand(1)},
  + {reduce_window->mutable_operand(1), operand},
  function));
  }
  