Make real sure we don't reassociate joins into or out of SEMI/ANTI joins.

 Make real sure we don't reassociate joins into or out of SEMI/ANTI joins.
 
 Per the discussion in optimizer/README, it's unsafe to reassociate anything
 into or out of the RHS of a SEMI or ANTI join.  An example from Piotr
 Stefaniak showed that join_is_legal() wasn't sufficiently enforcing this
 rule, so lock it down a little harder.
 
 I couldn't find a reasonably simple example of the optimizer trying to
 do this, so no new regression test.  (Piotr's example involved the random
 search in GEQO accidentally trying an invalid case and triggering a sanity
 check way downstream in clause selectivity estimation, which did not seem
 like a sequence of events that would be useful to memorialize in a
 regression test as-is.)
 
 Back-patch to all active branches.