Fast-path FK checks reject valid inserts for domain-typed FK columns

Hi,

Commit 2da86c1 ("Add fast path for foreign key constraint checks") makes
a foreign-key column whose type is a domain over a type different from
the referenced PK reject every valid row:

    CREATE DOMAIN int8dom AS int8;
    CREATE TABLE pk (a int4 PRIMARY KEY);
    CREATE TABLE fk (b int8dom REFERENCES pk(a));
    INSERT INTO fk VALUES (1);
    ERROR:  no conversion function from int8dom to integer

It's new in v19 (verified by building 2da86c1^, where the insert
succeeds); no released version is affected. The SPI path still handles
it fine, e.g. with a partitioned PK.

The fast path is the first caller to pass the cross-type pf_eq_oprs
operator to ri_HashCompareOp(). Its "no cast needed" test,

    if (typeid == righttype)

fails when the FK column is a domain over righttype (typeid is the
domain OID), so it wrongly concludes no conversion exists and errors out.
Looking through the domain fixes it -- conpfeqop is chosen against the
FK base type, so getBaseType(typeid) == righttype holds for any valid FK:

    -    if (typeid == righttype)
    +    if (getBaseType(typeid) == righttype)

Patch attached, with a regression test in foreign_key.sql. make check
and the isolation suite pass.

Regards,
Ewan Young