select_statement: Use precomputed value of _restrictions->need_filtering()
select_statement: Use precomputed value of _restrictions->need_filtering()

Instead of calculating _restrictions->need_filtering() each time,
we can now use the value that has been already calculated.

This used to happen during query execution,
so we get an increase in performance.

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

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