Actually there's a better way to do this, which is to count tuples

Actually there's a better way to do this, which is to count tuples
during the vacuumcleanup scan that we're going to do anyway.  Should
save a few cycles (one calculation per page, not per tuple) as well
as not having to depend on assumptions about heap and index being
in step.
I think this could probably be made to work for GIST too, but that
code looks messy enough that I'm disinclined to try right now.