Make the procedural shifts work with undefined shift values.
Make the procedural shifts work with undefined shift values.

This patch adds code to check for an undefined shift value in
the procedural opcodes. When an undefined value is found 'bx
is returned.

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