See Trac #14626, comment:4.  We want to maintain evaluted-ness
info on Ids into the code generateor for two reasons
(see Note [Preserve evaluated-ness in CorePrep] in CorePrep)

- DataToTag magic
- Potentially using it in the codegen (this is Gabor's
  current work)

But it was all being done very inconsistently, and actually
outright wrong -- the DataToTag magic hasn't been working for
years.

This patch tidies it all up, with Notes to match.