Adjust overly strict Assert

3373c715 changed how we determine EquivalenceClasses for relations and
added an Assert to ensure all relations mentioned in each EC's ec_relids
was a RELOPT_BASEREL.  However, the join removal code may remove a LEFT
JOIN and since it does not clean up EC members belonging to the removed
relations it can leave RELOPT_DEADREL rels in ec_relids.

Fix this by adjusting the Assert to allow RELOPT_DEADREL rels too.

Reported-by: sqlsmith via Andreas Seltenreich
Discussion: https://postgr.es/m/87y30r8sls.fsf@ansel.ydns.eu

src/backend/optimizer/path/equivclass.c

@@ -761,7 +761,8 @@ get_eclass_for_sort_expr(PlannerInfo *root,
 		{
 			RelOptInfo *rel = root->simple_rel_array[i];
 
-			Assert(rel->reloptkind == RELOPT_BASEREL);
+			Assert(rel->reloptkind == RELOPT_BASEREL ||
+				   rel->reloptkind == RELOPT_DEADREL);
 
 			rel->eclass_indexes = bms_add_member(rel->eclass_indexes,
 												 ec_index);