Fix duplicated constraints when using expressions

Fix duplicated constraints when using expressions

Fixed issue where using :meth:`.Operations.create_table` in conjunction
with a :class:`.CheckConstraint` that referred to table-bound
:class:`.Column` objects rather than string expressions would be added to
the parent table twice, resulting in an incorrect DDL sequence. Pull
request courtesy Nicolas CANIART.

Fixes: #1004
Closes: #1005
Pull-request: https://github.com/sqlalchemy/alembic/pull/1005
Pull-request-sha: 2fe5c5297bcde990096571a047039c451aa876f6

Change-Id: I2bf48701968fe59a6766f8f8879320b1bfd75774