Change more places to be less trusting of RestrictInfo.is_pushed_down.

On further reflection, commit e5d83995e didn't go far enough: pretty much
everywhere in the planner that examines a clause's is_pushed_down flag
ought to be changed to use the more complicated behavior where we also
check the clause's required_relids.  Otherwise we could make incorrect
decisions about whether, say, a clause is safe to use as a hash clause.

Some (many?) of these places are safe as-is, either because they are
never reached while considering a parameterized path, or because there
are additional checks that would reject a pushed-down clause anyway.
However, it seems smarter to just code them all the same way rather
than rely on easily-broken reasoning of that sort.

In support of that, invent a new macro RINFO_IS_PUSHED_DOWN that should
be used in place of direct tests on the is_pushed_down flag.

Like the previous patch, back-patch to all supported branches.

Discussion: https://postgr.es/m/f8128b11-c5bf-3539-48cd-234178b2314d@proxel.se

src/backend/optimizer/plan/initsplan.c

@@ -1775,6 +1775,11 @@ distribute_qual_to_rels(PlannerInfo *root, Node *clause,
 	 * attach quals to the lowest level where they can be evaluated.  But
 	 * if we were ever to re-introduce a mechanism for delaying evaluation
 	 * of "expensive" quals, this area would need work.
+	 *
+	 * Note: generally, use of is_pushed_down has to go through the macro
+	 * RINFO_IS_PUSHED_DOWN, because that flag alone is not always sufficient
+	 * to tell whether a clause must be treated as pushed-down in context.
+	 * This seems like another reason why it should perhaps be rethought.
 	 *----------
 	 */
 	if (is_deduced)