Partially undo commit
94da73281.
On closer inspection, mcv.c isn't as broken for ScalarArrayOpExpr
as I thought. The Var-on-right issue is real enough, but actually
it does cope fine with a NULL array constant --- I was misled by
an XXX comment suggesting it didn't. Undo that part of the code
change, and replace the XXX comment with something less misleading.