num_sa_scans in genericcostestimate

When costing a btree index scan, num_sa_scans gets computed twice, once in
btcostestmeate and once in genericcostestimate. But the computations are
different. It looks like the generic one includes all =ANY in any column
in the index, while the bt one includes only =ANY which or on columns for
which all the preceding index columns are tested for equality.

It looks like the generic one was there first then the bt-specific one was
added later to improve planning of btree indexes. But then shouldn't the
value be passed down to generic, rather than recomputed (differently)?
I've attached a patch to do that. Generic still computes the value itself
for other-than-btree indexes.

Or, is there a reason we want a different value to be used in
genericcostestimate?

The context for this is that I was looking at cases where btree indexes
were not using all the columns they could, but rather shoving some of the
conditions down into a Filter unnecessarily/unhelpfully. This change
doesn't fix that, but it does seem to be moving in the right direction.

This does cause a regression test failure due to an (apparently?)
uninteresting plan change.

Cheers,

Jeff