Modify additional power 2 calculations to use new helper functions

2nd pass of modifying various places which obtain the next power
of 2 of a number and make them use the new functions added in
f0705bb62.

In passing, also modify num_combinations(). This can be implemented
using simple bitshifting rather than looping.

Reviewed-by: John Naylor
Discussion: https://postgr.es/m/20200114173553.GE32763%40fetter.org

src/backend/statistics/mvdistinct.c

@@ -576,15 +576,7 @@ n_choose_k(int n, int k)
 static int
 num_combinations(int n)
 {
-	int			k;
-	int			ncombs = 1;
-
-	for (k = 1; k <= n; k++)
-		ncombs *= 2;
-
-	ncombs -= (n + 1);
-
-	return ncombs;
+	return (1 << n) - (n + 1);
 }
 
 /*