Fix bug generating code for shift of constant zero.
Fix bug generating code for shift of constant zero.

The code generator detects this case, but instead of returning the
constant zero, it breaks out and writes partial code for the shift.

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