DmdAnal: Kill `is_thunk` case in `splitFV`

The `splitFV` function implements the highly dubious hack
described in `Note [Lazy und unleashable free variables]` in
GHC.Core.Opt.DmdAnal. It arranges it so that demand signatures only
carry strictness info on free variables. Usage info is released through
other means, see the Note. It's purely for analysis performance reasons.

TODO: Update commit message

But in case of a strict FV demand, we still add the FV to the demand
signature *and* the lazy_fv DmdEnv. It would be far simpler (and more
precise) not having to split the demand over both places, if analysis
performance doesn't degrade too much.