Fixes SubArrayRef.get_begin_subscript(..)
- Fixed all the places where it was invoked. - get_begin_subscript(..) should be only called when generating code, so made sure that it is not being called at unnecessary places in :mod:`loopy`.
- Fixed all the places where it was invoked. - get_begin_subscript(..) should be only called when generating code, so made sure that it is not being called at unnecessary places in :mod:`loopy`.