Remove tests for Item::type() in Item_basic_value::eq()

This is a subtask for MDEV-14630, which can be done in a self-contained change.

Item_basic_value has a number of methods:

• is_basic_value(), which tests type() internally.
• null_eq()
• int_eq()
• str_eq()
• real_eq()

It's descendants:

• Item_param
• Item_null
• Item_int
• Item_double
• Item_float
• Item_string
  use the mentioned methods in eq().

Additionally, Item_param tests cmp_type(), which is bad for pluggable data types (because constants of different pluggable data types will be able to have the same cmp_type(), but they are not equal to each other).

This task is to change the code to be based on Type_handler methods instead of Item::type() and Item::cmp_type() tests.