Backported fix for CORE-4225 - Using COLLATE UNICODE_CI_AI in WHERE clause (not indexed) is extremely slow.

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/branches/B2_5_Release@58248 65644016-39b1-43b1-bf79-96bc8fe82c15