Use correct sign for signal and constant addition.
Use correct sign for signal and constant addition.

This was incorrectly using the expression sign not checking if
the two sub-expressions are both signed. This likely used to
work, but in the context of $signed which sets the expression
to signed so we can have a signed expression without both
sub-expressions being signed.

themperek/iverilog,CastMi/iverilog,themperek/iverilog,themperek/iverilog,CastMi/iverilog,CastMi/iverilog