MDEV-31479: Inconsistency between MRR and SQL layer costs can cause poor query plan

(Same as
TODO-3938: best_access_path shows negative costs for mrr=on)

best_access_path() assumes that quick select cost includes
(quick->rows/TIME_FOR_COMPARE) as a cost of checking the attached
part of the WHERE condition.

It calls adjust_quick_cost() to subtract addition from quick's cost.

The problem was that DS-MRR cost formula didn't include this cost.
For very large tables, adjust_quick_cost() would produce a negative
cost which would cause assert in debug build or bad query plan choice
in release builds.
Approved-by: Monty <monty@mariadb.org>

sql/multi_range_read.cc

@@ -2006,6 +2006,9 @@ bool DsMrr_impl::get_disk_sweep_mrr_cost(uint keynr, ha_rows rows, uint flags,
   /* Total cost of all index accesses */
   index_read_cost= primary_file->keyread_time(keynr, 1, rows);
   cost->add_io(index_read_cost, 1 /* Random seeks */);
+
+  cost->cpu_cost+= (rows2double(rows) / TIME_FOR_COMPARE +
+                   MULTI_RANGE_READ_SETUP_COST);
   return FALSE;
 }