This was noticed wben running "mtr --valgrind main.precedence"

The problem was that Item_func_like::escape could be left unitialized
when used with views combined with UNIONS like in:

create or replace view v1 as select 2 LIKE 1 ESCAPE 3 IN (SELECT 0 UNION SELECT 1), 2 LIKE 1 ESCAPE (3 IN (SELECT 0 UNION SELECT 1)), (2 LIKE 1 ESCAPE 3) IN (SELECT 0 UNION SELECT 1);

The above query causes in fix_escape_item()
escape_item->const_during_execution() to be true
and
escape_item->const_item() to be false

in which case 'escape' is never calculated.

The fix is to make the main logic of fix_escape_item() out to a
separate function and call that function once in Item.

Other things:
- Reorganized fields in Item_func_like class to make it more compact