Cranelift: Value labels for displaying IR

Being able to attribute textual labels to IR values that would then be shown in the textual IR representation generated by `display` would greatly benefit the readability of CLIR.

This could easily be implemented outside of cranelift, but not without rewriting pretty much all of the machinery in `cranelift_codegen::write`, which feels like a waste.

Implementation should be as easy as a `SecondaryMap`, for example.