Repair PG BIGSERIAL w/ TypeDecorator, Variant

Repair PG BIGSERIAL w/ TypeDecorator, Variant

Some of the dialect impl memoization for TypeDecorator
necessarily keeps the top-level TypeDecorator type
around, since a user-defined type will have bind and result
set processing behavior.  For both TypeDecorator and Variant,
PG dialect needs to ensure it's looking at the SQLAlchemy
type to check for SmallInteger / BigInteger.

Fixes: 3739
Change-Id: I2d45fb997f17c6272d6bb826a77d2dba665adae7
(cherry picked from commit 421fa6b8bf9f0c3c5041579c89ec405ce0f5e0b0)