We create Item_cache_* object for each operand for each left operand of
a subquery predicate. We also create Item_func_conv_charset for each string
constant that needs charset conversion. So here we have Item_cache wrapped
into Item_func_conv_charset.
When Item_func_conv_charset wraps an constant Item it gets it's value
in constructor. The problem is that Item_cache is ready to be used only
at execution time, which is too late.
The fix makes Item_cache wrapping constant to get ready at fix_fields() time.


mysql-test/r/subselect.result:
  test result
mysql-test/t/subselect.test:
  test case
sql/item_cmpfunc.cc:
  now Item_optimizer::fix_left will store constant arguments in cache
  on fix_fields() stage