Fix to the optimization of check-in [a47efb7c8520a011] that reads the values

Fix to the optimization of check-in [a47efb7c8520a011] that reads the values
of expressions used in an index-on-expression directly from the index rather
than recomputing the value.  If the expression has a top-level COLLATE or
unlikely() operator, be sure to clear the corresponding flags prior to
converting it into a TK_COLUMN expression.  Failure to do this is most likely
harmless in production, but might cause an assertion fault in debugging builds.
Ticket [b0cb8aff561a6dcd].  Test cases in TH3.

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