pushdown.
      
NDB supports only a limited set of item nodes for use in engine condition
pushdown. Because of this adding cache for const expression effectively
disabled this optimization.
      
The ndb_serialize_cond function is extended to support Item_cache and treat
it as a constant values.
A helper function called ndb_serialize_const is added. It is used to create
Ndb_cond value node from given const item.


mysql-test/suite/ndb/t/disabled.def:
  Bug#49746: Const expression caching led to NDB not using engine condition
  pushdown.
  Enabled ndb_condition_pushdown test after fixing appropriate bug.
sql/ha_ndbcluster_cond.cc:
  Bug#49746: Const expression caching led to NDB not using engine condition
  pushdown.
  The ndb_serialize_cond function is extended to support Item_cache and treat
  it as a constant values.
  A helper function called ndb_serialize_const is added. It is used to create
  Ndb_cond value node from given const item.
sql/item.cc:
  Bug#49746: Const expression caching led to NDB not using engine condition
  pushdown.
  The Item::cache_const_expr_analyzer function is adjusted to not create
  cache for Item_int_with_ref objects.
sql/item.h:
  Bug#49746: Const expression caching led to NDB not using engine condition
  pushdown.
  The result_type() method is added to Item_cache class.
  The Item_cache_str now initializes its collation.