clast.c: insert extra modulo guards involving loop iterator inside loop

clast.c: insert extra modulo guards involving loop iterator inside loop

This change is only relevant for the isl backend, where the domains
to be scanned may have existentially quantified variables.
Commit 57a3999baa11aff9af7101d66f884c5d14898dff (clast.c: insert_guard:
insert modulo guards based on existential variables first) ensured
that CLooG would generate the corresponding modulo guards.
However, all guards were inserted before the loop, while a modulo
constraint involving the loop iterator should of course be inserted
inside the loop.

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