clast.c: only update lower bounds during the construction of a lower bound

clast.c: only update lower bounds during the construction of a lower bound

The clast_minmax function is used both from within insert_guard_constraint
and insert_for.  It's only from within insert_for that a lower bound is
computed and that this lower bound should be updated.
In fact, within insert_guard_constraint, clast_minmax may be used
to compute a guard on a parameter and then the code for checking
whether an update is needed would access elements of the stride
array beyond its bounds.

Problem reported by Tobias Grosser <grosser@fim.uni-passau.de>.

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