Ensure @compiles calls down to the original compilation scheme

Ensure @compiles calls down to the original compilation scheme

Made a slight behavioral change in the ``sqlalchemy.ext.compiler``
extension, whereby the existing compilation schemes for an established
construct would be removed if that construct was itself didn't already
have its own dedicated ``__visit_name__``.  This was a
rare occurrence in 1.0, however in 1.1 :class:`.postgresql.ARRAY`
subclasses :class:`.sqltypes.ARRAY` and has this behavior.
As a result, setting up a compilation handler for another dialect
such as SQLite would render the main :class:`.postgresql.ARRAY`
object no longer compilable.

Fixes: #3732
Change-Id: If2c1ada4eeb09157885888e41f529173902f2b49