Summary:
Make `raiseIO#` produce `topRes` instead of `ExnRes`. `ExnRes`
leads to demand analysis being too aggressive, IMO, allowing
imprecise exceptions produced by `throw` to replace exceptions
thrown by `throwIO` that  would like to think of as precise.

This fixes that, but is certanly much more conservative than we
would ideally like. Let's see how bad it is.

Fixes Trac #13380

Reviewers: austin, bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3301