SI-8634 Improve asSeenFrom handling of existentials

Reverts the change to `asSeenFrom`, b7b81ca28. The test from that
commit remains; it must have been fixed in the right place in
the meantime.

The error that we've shed in `run/t8634.scala` also looks like
a progression.

Review by the build kitteh then by @adriaanm

If it seems like this is the right direction, I'll see about bisecting
what changed in the meantime. It is probably worth dbuilding this one,
too.