(Variant 4, with @@optimizer_adjust_secondary_key_costs, reuse in two
places, and conditions are replaced with equivalent simpler forms in two more)

In best_access_path(), ReuseRangeEstimateForRef-3,  the check
for whether
 "all used key_part_i used key_part_i=const"
was incorrect: it may produced a "NO" answer for cases when we
had:
 key_part1= const // some key parts are usable
 key_part2= value_not_in_join_prefix  //present but unusable
 key_part3= non_const_value // unusable due to gap in key parts.

This caused the optimizer to fail to apply ReuseRangeEstimateForRef
heuristics. The consequence is poor query plan choice when the index
in question has very skewed data distribution.

The fix is enabled if its @@optimizer_adjust_secondary_key_costs flag
is set.