The function remove_eq_cond removes the parts of a disjunction
for which it has been proved that they are always true. In the
result of this removal the disjunction may be converted into a 
formula without OR that must be merged into the the AND formula
that contains the disjunction.
The merging of two AND conditions must take into account the
multiple equalities that may be part of each of them.
These multiple equality must be merged and become part of the
and object built as the result of the merge of the AND conditions.
Erroneously the function remove_eq_cond lacked the code that 
would merge multiple equalities of the merged AND conditions.
This could lead to confusing situations when at the same AND 
level there were two multiple equalities with common members
and the list of equal items contained only some of these 
multiple equalities.
This, in its turn, could lead to an incorrect work of the
function substitute_for_best_equal_field when it tried to optimize
ref accesses. This resulted in forming invalid TABLE_REF objects
that were used to build look-up keys when materialized subqueries
were exploited.