Extend loose index scan to support descending indexes.
This is achieved by removing a block skipping creating loose index
scan plan for descending index, as well as generalising the execution
of such plans.
The generalisation applies to all levels looking for min/max in loose
index scan. In the highest level (get_next), generalise min and max to
first and last, so that it still proceeds in the direction agreeing
with the index parity. In the lower levels, combine next_min and
next_max methods into next_min_max, and combine next_min_in_range and
next_max_in_range into next_min_max_in_range. This retains existing
logic of these functions and reduces code duplication, while allowing
handling of all four combinations (min, max) x (asc index, desc
index).
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