Get rid of the planner's LateralJoinInfo data structure.

I originally modeled this data structure on SpecialJoinInfo, but after
commit acfcd45cac that looks like a pretty poor decision.
All we really need is relid sets identifying laterally-referenced rels;
and most of the time, what we want to know about includes indirect lateral
references, a case the LateralJoinInfo data was unsuited to compute with
any efficiency.  The previous commit redefined RelOptInfo.lateral_relids
as the transitive closure of lateral references, so that it easily supports
checking indirect references.  For the places where we really do want just
direct references, add a new RelOptInfo field direct_lateral_relids, which
is easily set up as a copy of lateral_relids before we perform the
transitive closure calculation.  Then we can just drop lateral_info_list
and LateralJoinInfo and the supporting code.  This makes the planner's
handling of lateral references noticeably more efficient, and shorter too.

Such a change can't be back-patched into stable branches for fear of
breaking extensions that might be looking at the planner's data structures;
but it seems not too late to push it into 9.5, so I've done so.

src/backend/optimizer/plan/analyzejoins.c

@@ -439,9 +439,6 @@ remove_rel_from_query(PlannerInfo *root, int relid, Relids joinrelids)
 		sjinfo->syn_righthand = bms_del_member(sjinfo->syn_righthand, relid);
 	}
 
-	/* There shouldn't be any LATERAL info to translate, as yet */
-	Assert(root->lateral_info_list == NIL);
-
 	/*
 	 * Likewise remove references from PlaceHolderVar data structures,
 	 * removing any no-longer-needed placeholders entirely.