Monotonic WindowFunc support for ntile(), percent_rank() and cume_dist()

9d9c02ccd [1] added infrastructure in the query planner and executor
so that the executor would know to stop processing a monotonic
WindowFunc when its value went beyond what some qual in the outer
query could possibly match in future evaluations due to the
WindowFunc's monotonic nature.

In that commit, support was added so that the optimisation would work
for row_number(), rank(), dense_rank() and forms of count(*).  On
further inspection, it looks like the same can be done for ntile(),
percent_rank() and cume_dist(). These WindowFuncs are always
monotonically increasing.

I've attached a trivial patch to add the required support request type
to the existing prosupport functions for these window functions.

David

[1] https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=9d9c02ccd