Bug description: Index db files do not contain the subtype knowledge,
which is only in the primary id2entry db and entries in the memory.
If the search filter includes subtype in the NOT condition and
the type is indexed, the condition is mistakenly simplified to
the one equivalent to not having the subtype.
E.g., if the given filter is (&(cn=A*)(!(cn;fr=ABC en)), it's
evaluated as (&(cn=A*)(!(cn=ABC en)).
Fix description: If a filter contains a subtype in NOT condition,
we give up using the index and leave the not evaluation to the
search return code.