Attach ON CONFLICT SET ... WHERE to the correct planstate.

The previous coding was a leftover from attempting to hang all the on
conflict logic onto modify table's child nodes. It appears to not have
actually caused problems except for explain.

Add test exercising the broken and some other code paths.

Author: Peter Geoghegan and Andres Freund

src/backend/executor/nodeModifyTable.c

@@ -1697,7 +1697,7 @@ ExecInitModifyTable(ModifyTable *node, EState *estate, int eflags)
 			ExprState  *qualexpr;
 
 			qualexpr = ExecInitExpr((Expr *) node->onConflictWhere,
-									mtstate->mt_plans[0]);
+									&mtstate->ps);
 
 			resultRelInfo->ri_onConflictSetWhere = (List *) qualexpr;
 		}

src/test/regress/expected/insert_conflict.out

@@ -43,6 +43,22 @@ explain (costs off) insert into insertconflicttest values(0, 'Crowberry') on con
    ->  Result
 (4 rows)
 
+explain (costs off) insert into insertconflicttest values(0, 'Crowberry') on conflict (key, fruit) do update set fruit = excluded.fruit
+  where exists (select 1 from insertconflicttest ii where ii.key = excluded.key);
+                                  QUERY PLAN                                   
+-------------------------------------------------------------------------------
+ Insert on insertconflicttest
+   Conflict Resolution: UPDATE
+   Conflict Arbiter Indexes: op_index_key, collation_index_key, both_index_key
+   Conflict Filter: (alternatives: SubPlan 1 or hashed SubPlan 2)
+   ->  Result
+   SubPlan 1
+     ->  Index Only Scan using both_index_expr_key on insertconflicttest ii
+           Index Cond: (key = excluded.key)
+   SubPlan 2
+     ->  Seq Scan on insertconflicttest ii_1
+(10 rows)
+
 -- Neither collation nor operator class specifications are required --
 -- supplying them merely *limits* matches to indexes with matching opclasses
 -- used for relevant indexes

src/test/regress/sql/insert_conflict.sql

@@ -20,6 +20,8 @@ explain (costs off) insert into insertconflicttest values(0, 'Crowberry') on con
 explain (costs off) insert into insertconflicttest values(0, 'Crowberry') on conflict (key, fruit) do nothing;
 explain (costs off) insert into insertconflicttest values(0, 'Crowberry') on conflict (fruit, key, fruit, key) do nothing;
 explain (costs off) insert into insertconflicttest values(0, 'Crowberry') on conflict (lower(fruit), key, lower(fruit), key) do nothing;
+explain (costs off) insert into insertconflicttest values(0, 'Crowberry') on conflict (key, fruit) do update set fruit = excluded.fruit
+  where exists (select 1 from insertconflicttest ii where ii.key = excluded.key);
 -- Neither collation nor operator class specifications are required --
 -- supplying them merely *limits* matches to indexes with matching opclasses
 -- used for relevant indexes