Shifting a signed constant should just return the L-value.
Shifting a signed constant should just return the L-value.

We already covered the case of shifting zero, but the case of
shifting a signed constant was missed.

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