pgsql: Fix expr_is_nonnullable() for IS DISTINCT FROM

Fix expr_is_nonnullable() for IS DISTINCT FROM

Commit c8d2f68cc taught expr_is_nonnullable() to treat a DistinctExpr
as never returning NULL. That is wrong: when neither input is NULL,
the result is NULL if the underlying equality operator returns NULL,
which nothing forbids. This could lead to wrong query results.

To fix, remove the DistinctExpr case, along with its test case.

While at it, test the relkind before calling has_subclass() in
var_is_nonnullable(), to avoid a useless syscache lookup for
partitioned tables.

Reported-by: Robert Haas <robertmhaas(at)gmail(dot)com>
Author: Richard Guo <guofenglinux(at)gmail(dot)com>
Discussion: https://postgr.es/m/CA+TgmoaLcsmc9weJr1yo1X1SwZJX1t4XXHj0SJzH8XztmN9Hdg@mail.gmail.com
Backpatch-through: 19

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f5635c2786af2322e1e21006a13a2c19f68fa591

Modified Files
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src/backend/optimizer/util/clauses.c | 12 ++----------
src/test/regress/expected/predicate.out | 10 ----------
src/test/regress/sql/predicate.sql | 4 ----
3 files changed, 2 insertions(+), 24 deletions(-)