JIT: revise checking for computed preds (#81582)

JIT: revise checking for computed preds (#81582)

Since pred lists now exist pervasively, change most methods that had
conditional pred list updates to assert preds exist and always update.

To make sure I got all uses, I renamed `fgComputePredsDone` to
`fgPredsComputed`.

Remove the ability to drop EH, as it doesn't update pred lists properly
and so has been broken for quite a while now.

Remove some of the logic for fixing up finally targets, since we now
always have pred lists around and so don't need the blanket invaliation.

Closes #80193.