Allow predicate_refuted_by() to deduce that NOT A refutes A.

We had originally made the stronger assumption that NOT A refutes any B
if B implies A, but this fails in three-valued logic, because we need to
prove B is false not just that it's not true.  However the logic does
go through if B is equal to A.

Recognizing this limited case is enough to handle examples that arise when
we have simplified "bool_var = true" or "bool_var = false" to just "bool_var"
or "NOT bool_var".  If we had not done that simplification then the
btree-operator proof logic would have been able to prove that the expressions
were contradictory, but only for identical expressions being compared to the
constants; so handling identical A and B covers all the same cases.

The motivation for doing this is to avoid unexpected asymmetrical behavior
when a partitioned table uses a boolean partitioning column, as in today's
gripe from Dominik Sander.

Back-patch to 8.2, which is as far back as predicate_refuted_by attempts to
do anything at all with NOTs.