opt_clean: handle undriven and x-bit driven bits consistently

[widlarizer]

I believe driving the bit of a signal with a constant x-bit is exactly the same as not driving it at all. This PR removes this inconsistency by extending SigPool. As the SigPool is used to check the assign_mapped wire bits against unmapped bits, this creates an arbitrary distinction between driven and undriven. I have added a pass-specific mechanism by inheriting from SigPool that happens to work for this case. I don't know if SigPool is similarly misused elsewhere.

This PR shares its motivation with #5003

[widlarizer]

This doesn't create the ideal behavior and the implementation is hard to defend either. Looking into alternatives

[KrystalDelusion]

Might be good to add some tests, in particular for checking that opt_clean -x and setundef -undriven -undef; opt_clean are equivalent.
Am I also right in saying that opt -full wouldn't benefit from this, since opt_expr -full (or more specifically, opt_expr -undriven) already replaces undriven bits with x-bits?

[KrystalDelusion]

Also given that setundef and opt_expr both use -undriven and refer to it as "replace undriven nets with undef (x) constants" in the help, it would be good to be consistent here (unless I'm misinterpreting, and this isn't the same).