The old code counted selectivity double in case of queries like:
WHERE key_part1=1 and key_part2 < 100
if the optimizer would decide to use a REF access on key_part1.

The new code in best_access_path() that changes REF access to RANGE
if the RANGE key is longer makes this issue less likely to happen.

I was not able to create a test case for 11.0, however if one ports this
patch to a MariaDB version without the change of REF to RANGE, the
selectivity will be counted double.