Add vacuum_delay_point call in compute_index_stats's per-sample-row loop.

Slow functions in index expressions might cause this loop to take long
enough to make it worth being cancellable.  Probably it would be enough
to call CHECK_FOR_INTERRUPTS here, but for consistency with other
per-sample-row loops in this file, let's use vacuum_delay_point.

Report and patch by Jeff Janes.  Back-patch to all supported branches.

src/backend/commands/analyze.c

@@ -738,6 +738,8 @@ compute_index_stats(Relation onerel, double totalrows,
 		{
 			HeapTuple	heapTuple = rows[rowno];
 
+			vacuum_delay_point();
+
 			/*
 			 * Reset the per-tuple context each time, to reclaim any cruft
 			 * left behind by evaluating the predicate or index expressions.