cql3: expr: possible_lhs_values(): open-code get_value_comparator()
cql3: expr: possible_lhs_values(): open-code get_value_comparator()

get_value_comparator() is going away soon, so open-code it here. It's
not doing much anyway.

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