Fix RI fast-path for domain-typed FK columns

Fix RI fast-path for domain-typed FK columns

The RI fast path is the first caller to pass a cross-type pf_eq_oprs
operator to ri_HashCompareOp().  Its test for whether a cast can be
skipped, "typeid == righttype", failed when the FK column was a domain,
since typeid is then the domain OID rather than its base type.  The code
concluded no usable conversion existed and threw "no conversion function
from <domain> to <type>" for every valid row.

Look through the domain to its base type.  When pfeqop comes directly
from the index opfamily its right-hand input is getBaseType(fktype), so
getBaseType(typeid) == righttype is the correct test; the PK = PK
fallback (right-hand input opcintype) still fails that test and falls
through to the existing cast lookup unchanged.

Oversight in commit 2da86c1.

Reported-by: Ewan Young <kdbase.hack@gmail.com>
Author: Ewan Young <kdbase.hack@gmail.com>
Reviewed-by: Amit Langote <amitlangote09@gmail.com>
Discussion: https://postgr.es/m/CAON2xHNDFC4cX2atvTpMuC=cK9y7q4J+n3+15w4148AohXEc1w@mail.gmail.com