Revert "`exprOkForSpeculation` for Note [IO hack in the demand analyser]"

This reverts commit ce64b397 on the
grounds of the regression it would introduce in a couple of packages.

Fixes #17653.

Also undoes a slight metric increase in #13701 introduced by that commit
that we didn't see prior to !1983.

Metric Decrease:
    T13701

(cherry picked from commit 071a58c6)