cql3: expr: Handle subscript in is_supported_by
cql3: expr: Handle subscript in is_supported_by

is_supported_by checks whether the given expression
is supported by some index.

The current behaviour seems wrong, but I kept
it to avoid making changes in a refactor PR.

Scylla doesn't have indexes on map entries yet,
so for a subscript the answer is always no.
I think we should just return false there.

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

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