sign extended expressions are signed.
sign extended expressions are signed.

Fix when sign extending an expression that the result is also signed.
This bug caused some arithmetic operations (with extended arguments)
to not notice that they are signed.

Also be more robust in noticing that comparisons are signed and should
generate the right code.
CastMi/iverilog,themperek/iverilog,CastMi/iverilog,themperek/iverilog,themperek/iverilog,CastMi/iverilog