[xc7] Invalid usage of FAN[0-7] or BYP[0-7] nodes

[litghost]

Currently the router can use FAN[0-7] and BYP[0-7] nodes in a way that generates antennee nets. In particular, if the FAN[0-7] and BYP[0-7] connect to a site pin, that node should only be used if the router is visiting the site pin. Current the router thinks the connections between the FAN/BYP routing node to the IPIN is configurable, but this is not the case.

Solution is unclear, but likely involves making the FAN/BYP into IPIN nodes, and collapsing the nodes in between.

[litghost]

Vaughn had a good suggestion to address this:

1. Because FAN_ALT -> FAN -> IPIN are non-configurable, update these VPR switchs to be buffering /non-configurable
2. Add a pass after net terminals are assigned to set the occupancy of all unused IPINs (and their associated non-configurable sets) to 1. This will force the router to avoid these rr nodes.