Make Item_func_eq of the following forms sargable by updating the relevant range
analysis methods:
1. substr(col, 1, n) = str
2. str = substr(col, 1, n)
3. left(col, n) = str
4. str = left(col, n)
where col is a indexed column and str is a const and inexpensive item
of length n.
We do this by factoring out Item_func_like::get_mm_leaf() and apply it
to a string obtained from escaping str and then appending a wildcard
"%" to it.
The addition of the two Functype enums, LEFT_FUNC and SUBSTR_FUNC,
requires changes in the spider group by handler to continue handling
LEFT and SUBSTR correctly.
Co-authored-by: Yuchen Pei <ycp@mariadb.com>
Co-authored-by: Sergei Petrunia <sergey@mariadb.com>
