Expression indexes can't benefit from pre-computed statistics on
columns.

Reported-by: Nikolay Samokhvalov

Discussion: https://postgr.es/m/CANNMO++5rw9RDA=p40iMVbMNPaW6O=S0AFzTU=KpYHRpCd1voA@mail.gmail.com

Author: Nikolay Samokhvalov, modified

Backpatch-through: 9.5