Note [Adding evaluatedness info to pattern-bound variables]

addEvals records the evaluated-ness of the bound variables of
a case pattern.  This is *important*.  Consider

     data T = T !Int !Int

     case x of { T a b -> T (a+1) b }

We really must record that b is already evaluated so that we don't
go and re-evaluate it when constructing the result.
See Note [Strict fields in Core] in GHC.Core.

NB: simplLamBndrs preserves this eval info

In addition to handling data constructor fields with !s, addEvals
also records the fact that the result of seq# is always in WHNF.
See Note [seq# magic] in GHC.Types.Id.Make.  Example (#15226):

  case seq# v s of
    (# s', v' #) -> E

we want the compiler to be aware that v' is in WHNF in E.

Open problem: we don't record that v itself is in WHNF (and we can't
do it here).  The right thing is to do some kind of binder-swap;
see #15226 for discussion.