Cycles converts internal links to converter nodes which don't do anything and
later on get collapsed by the graph optimization. However, the previous code
assumed that one Blender input socket maps to one Cycles input socket.
When a node is muted, there might be internal links from one input to multiple
outputs. In Cycles, this meant that one Blender input socket now mapped to
multiple input sockets on all the converter nodes, so only the last one survived.
THe fix is simple, just make the mapping a MultiMap.
2b4bf586ad