Fixed a couple of bugs in transitive reduction:

   1. Only consider paths through nodes reachable from the source in the inner loop.
   2. Ignore paths through Merge nodes, since we cannot know which branch executed.
   3. Ignore function nodes, since we are potentially mixing paths from multiple instantiations.
   4. Process control inputs to be removed in reverse order of input slot, so we don't clobber the repeated field.

PiperOrigin-RevId: 179596846