Note [Anticipating ANF in demand analysis]

When analysing non-complex (e.g., trivial) thunks and complex function
arguments, we have to pretend that the expression is really in administrative
normal form (ANF), the conversion to which is done by CorePrep.

Consider
```
f x = let y = x |> co in y `seq` y `seq` ()
```
E.g., 'y' is a let-binding with a trivial RHS. That may occur if 'y' can't be
inlined, for example. Now, is 'x' used once? It may appear as if that is the
case, since its only occurrence is in 'y's memoised RHS. But actually, CorePrep
will *not* allocate a thunk for 'y', because it is trivial and could just
re-use the memoisation mechanism of 'x'! By saying that 'x' is used once it
becomes a single-entry thunk and a call to 'f' will evaluate it twice.
The same applies to trivial arguments, e.g., `f z` really evaluates `z` twice.

So, somewhat counter-intuitively, trivial arguments and let RHSs will *not* be
memoised. On the other hand, evaluation of non-trivial arguments and let RHSs
*will* be memoised. In fact, consider the effect of conversion to ANF on complex
function arguments (as done by 'GHC.Core.Prep.mkFloat'):
```
f2 (g2 x) ===> let y = g2 x in f2 y                   (if `y` is lifted)
f3 (g3 x) ===> case g3 x of y { __DEFAULT -> f3 y }   (if `y` is not lifted)
```
So if a lifted argument like `g2 x` is complex enough, it will be memoised.
Regardless how many times 'f2' evaluates its parameter, the argument will be
evaluated at most once to WHNF.
Similarly, when an unlifted argument like `g3 x` is complex enough, we will
evaluate it *exactly* once to WHNF, no matter how 'f3' evaluates its parameter.

Note that any evaluation beyond WHNF is not affected by memoisation. So this
Note affects the outer 'Card' of a 'Demand', but not its nested 'SubDemand'.
'anticipateANF' predicts the effect of case-binding and let-binding complex
arguments, as well as the lack of memoisation for trivial let RHSs.
In particular, this takes care of the gripes in
Note [Analysing with absent demand] relating to unlifted types.