Remove SWAP gates from the two-qubit gate whitelist for Staq placement
also modified in f1eedf04. Otherwise, Swap instructions between
non-adjacent qubits can slip through the swap-shortest-path
IRTransformation. An example of this would be the QCOR qft() routine,
which can generate a Swap between far-away qubits at the end.

Combined with f1eedf04

, fixes ORNL-QCI/qcor#199.

Signed-off-by: Austin Adams <aja@gatech.edu>