matching_candidates_in_table() computes the number of rows one
gets from the current table after applying the WHERE clause on
just this table

The function had a "found_counstraint heuristic" which reduced the
number of rows after WHERE check by 25% if there were comparisons
between key parts in table T and previous tables, like WHERE
T.keyXpartY= func(prev_table.cols)

Note that such comparisons can only be checked when the row of
table T is joined with rows of the previous tables. It is wrong
to apply the selectivity before the join operation.

Fixed by moving the 'found_constraint' code to a separate function
and only reducing the #rows in 'records_out'.

Renamed matching_candidates_in_table() to apply_selectivity_for_table() as
the function now either applies selectivity on the rows (depending
on the value of thd->variables.optimizer_use_condition_selectivity)
or uses the selectivity from the available range conditions.