Fix misprocessing of equivalence classes involving record_eq().

canonicalize_ec_expression() is supposed to agree with coerce_type() as to
whether a RelabelType should be inserted to make a subexpression be valid
input for the operators of a given opclass.  However, it did the wrong
thing with named-composite-type inputs to record_eq(): it put in a
RelabelType to RECORDOID, which the parser doesn't.  In some cases this was
harmless because all code paths involving a particular equivalence class
did the same thing, but in other cases this would result in failing to
recognize a composite-type expression as being a member of an equivalence
class that it actually is a member of.  The most obvious bad effect was to
fail to recognize that an index on a composite column could provide the
sort order needed for a mergejoin on that column, as reported by Teodor
Sigaev.  I think there might be other, subtler, cases that result in
misoptimization.  It also seems possible that an unwanted RelabelType
would sometimes get into an emitted plan --- but because record_eq and
friends don't examine the declared type of their input expressions, that
would not create any visible problems.

To fix, just treat RECORDOID as if it were a polymorphic type, which in
some sense it is.  We might want to consider formalizing that a bit more
someday, but for the moment this seems to be the only place where an
IsPolymorphicType() test ought to include RECORDOID as well.

This has been broken for a long time, so back-patch to all supported
branches.

Discussion: https://postgr.es/m/a6b22369-e3bf-4d49-f59d-0c41d3551e81@sigaev.ru