Fix propagation of unforced bits when forcing a part select.
Fix propagation of unforced bits when forcing a part select.

vvp_net_t::force_vec4 propagates all bits of the forced value passed
to it, regardless of the mask value. I can't see any way to fix this
directly, so instead make sure anything that calls force_vec4 sets
the unforced bits of the passed value to the correct value.

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