I added this "optimization" on top of Amit Langote's 158b7bc6, but
the quick path is never taken because the partition uses a different
pg_type oid than its parent table (causing equalTupleDescs to return
false).  Changing that requires more analysis and is too considered
dangerous at this point in the cycle, so revert it.

We might make it work someday, but not for pg11.

Discussion: https://postgr.es/m/825031be-942c-8c24-6163-13c27f217a3d@lab.ntt.co.jp