`exprOkForSpeculation` for Note [IO hack in the demand analyser]
In #14998 (closed) I realised that the notion of speculative execution exactly matches eager evaluation of expressions in a case alternative where the scrutinee is an IO action.
Normally we have to deferIO any result from that single case
alternative to prevent this speculative execution, so we had a special
case in place in the demand analyser that would check if the scrutinee
was a prim-op, in which case we assumed that it would be ok to do the
eager evaluation.
Now we just check if the scrutinee is exprOkForSpeculation,
corresponding to the notion that we want to push evaluation of the
scrutinee after eagerly evaluating stuff from the case alternative.
This fixes #14998 (closed), because it resolves the last open Item 4 there.