Do not allow the query planner to be tricked into thinking that an index on a

Do not allow the query planner to be tricked into thinking that an index on a
constant expression might be useful for something.  Problem reported on
[forum:/forumpost/ecdfc02339|forum post ecdfc02339].  This is a follow-up
to the fixes at [44200596aa943963] and [2d2b91cc0f6fed8c].

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