Do not use subquery result, when value is unknown, for constant folding.
v2: fix simple subqueries, fixes 00597_push_down_predicate.
v3:
- use identity over introducing yet another cast analog (as suggested by @akuzm)
- simpler suitable_for_const_folding check
v4: use identity(cast()) since only cast() can provide corrent type (for
data types that does not have it's own type, i.e. DateTime)
v5: do not optimize consts if only_analyze isset, regardless the block
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