cql3: expr: Handle subscript in prepare_expression
cql3: expr: Handle subscript in prepare_expression

column_value can't be prepared, so subscript can't be prepared as well.

Signed-off-by: Jan Ciolek <b66a6f3989e26cb552db14b7ab163c3a50292e24@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla