narrow the check for double-paren exprs in mysql create_index

narrow the check for double-paren exprs in mysql create_index

Fixed regression from SQLAlchemy 1.3.20 caused by the fix for
:ticket:`5462` which adds double-parenthesis for MySQL functional
expressions in indexes, as is required by the backend, this inadvertently
extended to include arbitrary :func:`_sql.text` expressions as well as
Alembic's internal textual component,  which are required by Alembic for
arbitrary index expressions which don't imply double parenthesis.  The
check has been narrowed to include only binary/ unary/functional
expressions directly.

Fixes: #5800
Change-Id: I40f83c6f9dd04b984d0c86eba632a588570709a1