clast construction: keep track of valid stride entries

clast construction: keep track of valid stride entries

insert_modulo_guard_constraint tries to exploit strides of outer
loops to simplify the constraint.  However, insert_modulo_guard_constraint
may have been called from within insert_extra_modulo_guards, in which
case the "level" may be strictly greater than the current loop level,
i.e., the number of valid entries in the strides array.
We therefore need to explicitly keep track of the number of valid
entries in that array.

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>