Record evaluated-ness information correctly for strict constructors

Record evaluated-ness information correctly for strict constructors

The add_evals code in Simplify.simplAlt had bit-rotted.  Example:

  data T a = T !a
  data U a = U !a

  foo :: T a -> U a
  foo (T x) = U x

Here we should not evaluate x before building the U result, because
the x argument of T is already evaluated.

Thanks to Roman for finding this.