Can we rely on the ordering of paths in pathlist?

As the comment above add_path() says, 'The pathlist is kept sorted by
total_cost, with cheaper paths at the front.' And it seems that
get_cheapest_parallel_safe_total_inner() relies on this ordering
(without being mentioned in the comments, which I think we should do).
I'm wondering if this is the right thing to do, as in other places
cheapest total cost is found by compare_path_costs(), which would
consider startup cost if paths have the same total cost, and that seems
more sensible.

Attach a trivial patch to make get_cheapest_parallel_safe_total_inner
act this way.

Thanks
Richard