Make impossible checks smarter about IfAlreadyPresent

[rydrman]

You are right, there doesn't have to be real request. A combined IfAlreadyPresent request that was impossible here, but never got a real request, could prevent a valid solution, when generating it is the only option. Anecdotally, this doesn't seem to happen much, which could be our kinds of solves. Intuitively, these kinds of IfAlreadyPresent combined requests are not possible so satisfy, so you'd hope the solver would not spend time searching beyond them. This approach stops it doing that, but can leave a hole in the search that might matter so it's not ideal.

Just to make another quick note, this is my main concern with the approach here. I don't see it as a viable solution to have the solver miss possible valid solutions

Originally posted by @rydrman in #884 (comment)