This actually, fix for the patch for bug-27354. The problem with
the patch was that Item_func_sp::used_tables() was updated, but
Item_func_sp::const_item() was not. So, for Item_func_sp, we had
the following inconsistency:
  - used_tables() returned RAND_TABLE, which means that the item
    can produce "random" results;
  - but const_item() returned TRUE, which means that the item is
    a constant one.

The fix is to change Item_func_sp::const_item() behaviour: it must
return TRUE (an item is a constant one) only if a stored function
is deterministic and each of its arguments (if any) is a constant
item.


mysql-test/r/sp.result:
  Update result file.
mysql-test/t/sp.test:
  Add test cases for BUG-31035, BUG-31191, BUG-31226
sql/item_func.cc:
  Make Item::const_item() be consistent with Item::used_tables().