Optimize Pass Through nodes for unused inputs

[RunDevelopment]

Motivation
As I pointed out in #2549, with Pass Through supporting multiple inputs, dead nodes can no longer be effectively eliminated. The issue is that unused inputs cannot be detected by dead node elimination, leading to unnecessary computation.

Description
Add an optimization that removers unused inputs from Pass Through nodes.

Careful! This optimization has to be implemented in a way that does not destroy happens-before relationships. Since the upstream nodes of an unused input may have side effects, removing the unused input may change the order in which those side effects are executed.

Since we currently do not have side-effect nodes with outputs, please pretend that the Invert Color node has side effects in the following example:

In the above chain, Invert Color is guaranteed to be executed before Save Image. However, by removing the used input, the Invert Color and Save Image nodes have no relationship to each other anymore and chainner may execute them in any order.

See #2556.