isl_ast_build_ast_from_schedule: also add stride guard in generic case

isl_ast_build_ast_from_schedule: also add stride guard in generic case

Any guard implied by the stride constraint in the AST build domain
needs to be enforced by the generated AST and may not necessarily
be implied by the strided loop itself.
We were already adding this guard to the graft in the degenerate case,
but not in the generic case.

The implied guard is usually already available in loop bounds constraints,
so it is not clear if this fixes any bug that can appear in practice,
However, this may just be a consequence of the internal representation of sets
and we should not depend too much on this internal representation.

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