[TGT-TTB] Bug Fixes with Propagation

I made some bad assumptions about how propagation works with nexuses. (Such as assuming the first element must be the
signal we are propagating off of). This will cause a lot of missed connection which we now find.

Also found that any time a nexus is connected to two signals, it means those signals are on the same wire (for example,
one maybe inside a module while the other is outside)
Merge branch 'spqr' of github.com:schultetwin1/iverilog into spqr

Conflicts:
	tgt-ttb/propagate-lpm.cc