cql3: Use a variable instead of dynamic cast
cql3: Use a variable instead of dynamic cast

There is a dynamic cast used to determine whether
clustering columns are restricted by a multi column
restriction.

Instead of doing that we can just use the _has_multi_column
variable.

It's also used a few lines higher, which means that
it should be already initialized.

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

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