vulkan: sort graph to allow more parallel execution (#15850)

vulkan: sort graph to allow more parallel execution (#15850)

* vulkan: sort graph to allow more parallel execution

Add a backend proc to allow the backend to modify the graph. The
vulkan implementation looks at which nodes depend on each other
and greedily reorders them to group together nodes that don't
depend on each other. It only reorders the nodes, doesn't change
the contents of any of them.

With #15489, this reduces the number of synchronizations needed.

* call optimize_graph per-split