Use default copy constructor for RTLIL::SigBit

Use default copy constructor for RTLIL::SigBit

There was a handwritten copy constructor, which I'm not sure was
actually legal C++ (it unconditionally read from the 'data' member of
a union, which wouldn't have been written if wire was true). It was
also a bit less efficient than the constructor you get from the
compiler by default (which is allowed to just copy the memory).

This gives a marginal (~0.25%) decrease in code size when compiled
with GCC 9.3.