Flip-flops with a direct output cause combinatorial loops with Antisat

[Coloquinte]

This can be seen with the following test:

yosys -m moosic -p "read_blif -sop blif/iscas89-s1196a.blif; flatten; synth; show; logic_locking -target outputs -nb-antisat 16 -antisat antisat -key 77777777777777777777777777777777777777; check"

[Coloquinte]

What happens is that:

1. We decide which gates to lock - the flip-flop in this case
2. Add the Antisat structures, using the flip-flop output
3. Add key gates and connect them to the Antisat, creating the loop

The issue is that the creation of the Antisat structure uses the inputs before locking, but locking adds key gates before some of those inputs. We could:

• Forbid locking on any combinatorial input, at least when using Antisat
• Create the Antisat structure after instanciating the key gates

[Coloquinte]

Now fixed: locking is done first, then the Antisat structure, then the connections between them