Fix failure to set correct operator in window run condition

Fix failure to set correct operator in window run condition

This was a simple omission in 9d9c02ccd where the code didn't correctly
set the operator to use in the run condition OpExpr when the window
function was both monotonically increasing and decreasing.

Bug discovered by Julien Roze, although he did not report it.

Reported-by: Phil Florent
Discussion: https://postgr.es/m/PA4P191MB160009A09B9D0624359278CFBA9F9@PA4P191MB1600.EURP191.PROD.OUTLOOK.COM
Backpatch-through: 15, where 9d9c02ccd was added