Draft: Demand Analysis: Better Product demands on scrutinees (#18907)
In #18907 (closed) and #16859 (closed) we listed quite a few cases in which demand
analysis attaches Product demands to binders when the body doesn't
actually use the fields, causing WW to unbox some arguments when it
better shouldn't. It's all in Note [Scrutinee demands and unboxing]
in GHC.Types.Demand.
Fixes #18907 (closed) and #16859 (closed).