Backport additional fix for bug CORE-4302 : Lookup (or scan) in descending index could be very inefficient for some keys

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/branches/B2_5_Release@59006 65644016-39b1-43b1-bf79-96bc8fe82c15
Backport fix for bug CORE-2165 : Unnecessary 1 index read may occur when using strict inequality condition

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/branches/B2_5_Release@59007 65644016-39b1-43b1-bf79-96bc8fe82c15