Improve planner's understanding of strictness of type coercions.

PG type coercions are generally strict, ie a NULL input must produce
a NULL output (or, in domain cases, possibly an error).  The planner's
understanding of that was a bit incomplete though, so improve it:

* Teach contain_nonstrict_functions() that CoerceViaIO can always be
considered strict.  Previously it believed that only if the underlying
I/O functions were marked strict, which is often but not always true.

* Teach clause_is_strict_for() that CoerceViaIO, ArrayCoerceExpr,
ConvertRowtypeExpr, CoerceToDomain can all be considered strict.
Previously it knew nothing about any of them.

The main user-visible impact of this is that IS NOT NULL predicates
can be proven to hold from expressions involving casts in more cases
than before, allowing partial indexes with such predicates to be used
without extra pushups.  This reduces the surprise factor for users,
who may well be used to ordinary (function-call-based) casts being
known to be strict.

Per a gripe from Samuel Williams.  This doesn't rise to the level of
a bug, IMO, so no back-patch.

Discussion: https://postgr.es/m/27571.1550617881@sss.pgh.pa.us

src/backend/optimizer/util/clauses.c

@@ -1172,6 +1172,16 @@ contain_nonstrict_functions_walker(Node *node, void *context)
 		return true;
 	if (IsA(node, FieldStore))
 		return true;
+	if (IsA(node, CoerceViaIO))
+	{
+		/*
+		 * CoerceViaIO is strict regardless of whether the I/O functions are,
+		 * so just go look at its argument; asking check_functions_in_node is
+		 * useless expense and could deliver the wrong answer.
+		 */
+		return contain_nonstrict_functions_walker((Node *) ((CoerceViaIO *) node)->arg,
+												  context);
+	}
 	if (IsA(node, ArrayCoerceExpr))
 	{
 		/*

src/backend/optimizer/util/predtest.c

@@ -1352,6 +1352,27 @@ clause_is_strict_for(Node *clause, Node *subexpr)
 		return false;
 	}
 
+	/*
+	 * CoerceViaIO is strict (whether or not the I/O functions it calls are).
+	 * Likewise, ArrayCoerceExpr is strict for its array argument (regardless
+	 * of what the per-element expression is), ConvertRowtypeExpr is strict at
+	 * the row level, and CoerceToDomain is strict too.  These are worth
+	 * checking mainly because it saves us having to explain to users why some
+	 * type coercions are known strict and others aren't.
+	 */
+	if (IsA(clause, CoerceViaIO))
+		return clause_is_strict_for((Node *) ((CoerceViaIO *) clause)->arg,
+									subexpr);
+	if (IsA(clause, ArrayCoerceExpr))
+		return clause_is_strict_for((Node *) ((ArrayCoerceExpr *) clause)->arg,
+									subexpr);
+	if (IsA(clause, ConvertRowtypeExpr))
+		return clause_is_strict_for((Node *) ((ConvertRowtypeExpr *) clause)->arg,
+									subexpr);
+	if (IsA(clause, CoerceToDomain))
+		return clause_is_strict_for((Node *) ((CoerceToDomain *) clause)->arg,
+									subexpr);
+
 	return false;
 }