s390x/tcg: MVC: Use is_destructive_overlap()

Let's use the new helper, that also detects destructive overlaps when
wrapping.

We'll make the remaining code (e.g., fast_memmove()) aware of wrapping
later.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>