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