cxxrtl: only write VCD values that were actually updated.
cxxrtl: only write VCD values that were actually updated.

On a representative design (Minerva SoC) this reduces VCD file size
by ~20× and runtime by ~3×.

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