Fix optimization of base==0 part select
Fix optimization of base==0 part select

Some patch caused a variable that held the constant base of a part
select to become unassigned. This led to unexplained failures to part
select a signal. Fix by properly detecting the case of a constant base
of 0 for the part select.
themperek/iverilog,themperek/iverilog,CastMi/iverilog,themperek/iverilog,CastMi/iverilog,CastMi/iverilog