Internaly in the storage engines an enum data type is represented by the
'index' into the textual representation of values it may accept.
Thus the SE knows nothing about which textual vales the stored enum value
represents.

As Enums are expected to be compared by their textual 'string' value,
this comparison can't be handled by the storage engine.

This patch eliminate condition pushdown of enum values when they
are used in a precicate where they are compared '>' or '<'.
We still allow enums to be pushed down in conditions where they
are compared '=' or '<>' as such comparison are first converted
from their textual to their index value.