Fix conversion of negative integer to real.
Fix conversion of negative integer to real.

When an integer/bool value is constant and negative,
and is used in a real-valued context, fix the
code generator calculation of the negative value.

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