(How to) Determine bit width and signedness for `Int` values from `Discriminant`

[jberthold]

#792 sets bitwidth of Discriminant values to 0 as a sentinel and relaxes type constraints for comparison and SwitchInt.
If possible, we want to instead use the correct bit width and signedness for discriminants but it is unclear whether and how they can be obtained from the layout (it is not the tag encoding alone, or else the MIR tolerates type mismatches for assignments).