write_cxxrtl: ignore disconnected module ports.
write_cxxrtl: ignore disconnected module ports.

E.g. port `q` in `submod x(.p(p), .q());`.

Fixes #1920.

antmicro/yosys,cliffordwolf/yosys,cliffordwolf/yosys,YosysHQ/yosys,antmicro/yosys,YosysHQ/yosys,SymbiFlow/yosys,antmicro/yosys,YosysHQ/yosys,cliffordwolf/yosys,antmicro/yosys,antmicro/yosys,YosysHQ/yosys,SymbiFlow/yosys,antmicro/yosys,cliffordwolf/yosys,SymbiFlow/yosys,cliffordwolf/yosys,SymbiFlow/yosys,SymbiFlow/yosys,SymbiFlow/yosys,antmicro/yosys,YosysHQ/yosys,antmicro/yosys,cliffordwolf/yosys,SymbiFlow/yosys,cliffordwolf/yosys,cliffordwolf/yosys,cliffordwolf/yosys,YosysHQ/yosys,YosysHQ/yosys,SymbiFlow/yosys,YosysHQ/yosys