Improve error message when shifting int by non-const

[vlstill]

Fixes #4652. Note that besides changing the error I've also allowed the case when the right operand is bit<N>/int<N> but it is also constant. This case should be allowed and it does actually work in most cases in practice, it is just covered by constant folding.

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BTW: I did not realize until now that the type checker actually does constant folding too! This will make any "small frontend" quite hard to do if it was to not do constant folding.

[fruffy]

BTW: I did not realize until now that the type checker actually does constant folding too! This will make any "small frontend" quite hard to do if it was to not do constant folding.

Yes, I believe this is necessary to fold compile-time arithmetic in bit<> types. At least the constantFolding pass is relatively self-contained and can be applied without a type or ref-map. We could fork out a smaller version that only works on compile-time constants and is used for the purpose of type inference.

It could then be extended by the full ConstantFolding pass.