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
f0705bb6.

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