Fix for br978 - assertion involving addition of $ivlh_to_unsigned() result.
Fix for br978 - assertion involving addition of $ivlh_to_unsigned() result.

$ivlh_to_unsigned, unlike $signed and $signed, can cause a reduction
in width. The shared PECallFunction::cast_to_width_ method did not
support this.

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