clast.c: insert_guard: insert modulo guards based on existential variables first

clast.c: insert_guard: insert modulo guards based on existential variables first

In the isl backend, the domains to be scanned may have existentially
quantified variables.  During the loop generation, we do not recurse
into these variables, i.e., there is no "level" corresponding to these
variables.  This means that insert_equation and the insert_modulo_guard
within that function will never be called for these variables,
while existential variables are expressly used to encode modulo constraints.

We therefore introduce a check for modulo constraints imposed by any
existential variable within each call to insert_guard.

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