Improve performance of HloComputation::MakeInstructionPostOrder

Previously it used the same infrastructure as HloInstruction::Accept
what caused a high overhead for large models due to the excess amount of
work it have to do to support modifying the graph under iteration and due
to the lack of caching on graphs with multiple sinks.

The new code is a very simple implementation of an iterative DFS based
topological sort.

PiperOrigin-RevId: 199606688