If the final range restrictions (SEL_ARG tree) over GROUP BY
columns are single-point, we can compute the number of GROUP BY groups.

Example: in the query:
SELECT ... FROM tbl
WHERE keypart1 IN (1,2,3) and keypart2 IN ('foo','bar')

Other things:
- Fixed cost calculation to more correctly count the number of blocks
  that may be read. The old code could use the total blocks in the file
  even if a range was available.