SI-7006 Simplify jump-only block destination determination

With proper reachability analysis, the code for finding the final
destination of jump-only blocks was more complicated than needed. This
commit simplifies and speeds up that process using a standard
Tortoise and Hare algorithm on a Map from jump-only blocks to their
immediate destinations.

Test t7006 is increased a bit to make sure we don't get stuck on
infinite loops and to make sure we're deleting all but the essential
jump.