Fix floordiv (//) for float/numeric by int with div_is_floordiv dialects

Fixed issue where floor division (``//``) between a :class:`.Float` or
:class:`.Numeric` numerator and an :class:`.Integer` denominator would omit
the ``FLOOR()`` SQL wrapper on dialects where
:attr:`.Dialect.div_is_floordiv` is ``True`` (the default, including
PostgreSQL and SQLite).  ``FLOOR()`` is now applied if either the
denominator or the numerator is a non-integer, so that expressions such as
``float_col // int_col`` render as ``FLOOR(float_col / int_col)`` instead
of the incorrect ``float_col / int_col``.  Pull request courtesy r266-tech.

Fixes: #10528

Closes: #13191
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/13191
Pull-request-sha: c9cbc47c877e19c91f912556b4ead6cd26e3cfe6

Change-Id: I5f9f02d966aa6ccee214a2c5cc27a73a4292da03