[FASM] Fixed Bug With Wire Creation

[AlexandreSinger]

Found a bug within FASM's wire generation where it uses the index of the output pin to create the wire instead of the index of the input pin.

This stemmed from some confusing code which both verified that the wire was being used and returning the first valid pin. It just so happens that it checked the outputs first and returned the output pin instead.

Cleaned up the code and added more error checking to prevent issues like this in the future.

[AlexandreSinger]

@vaughnbetz , when upgrading to the most recent version of VTR, our testing suite at ZeroASIC found this issue. We simulated the FASM generated by genfasm and found that the lookup table for the wire implementation was incorrect. Based on this issue, we believe that the first inputs to wires were always being used; however, due to recent changes to the packer, we may have allowed different inputs of "wire" mode LUTs to be used which exposed this issue.

This PR fixes the issue and adds some self-checking code to try and prevent headache in the future. Please review when you have a moment.

[amin1377]

Thanks, Alex! I know you didn’t write the code, but I thought you might have some insight into my questions. For both get_wire_input_pin and is_wire_used, based on the input parameters, I’m not quite sure what the term "wire" refers to. If you could help clarify that, I’d really appreciate it.