WHERE F(X) = constY should used F(X) generated index and query WHERE G=constY

I noticed when porting MDEV-15854 that in MySQL:

https://github.com/mysql/mysql-server/blob/8.0/mysql-test/r/func_uuid.result#L297-L301

There wasn't any specific UUID code causing this so it could be generic.

For any pure function, a generated index on F(X) for a query `WHERE F(X) = Y` could used the generated index `G=F(X)`to search for `Y``.

There's an overhead of course searching for a generated column`F(X)` in a query, however this only needs to be done if the comparison point is a constant and the other side is a function. When this occurs no index is usable anyway.