Fix for br965.
Fix for br965.

When performing constant propagation, we need to take into account
values driven through a tran object. For now, be pessimistic, and
assume that all tran objects connect to a variable driver.

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