xen/sched: we never get into context_switch() with prev==next

xen/sched: we never get into context_switch() with prev==next

In schedule(), if we pick, as the next vcpu to run (next) the same one
that is running already (prev), we never get to call context_switch().

We can, therefore, get rid of all the `if`-s testing prev and next being
different, trading them with an ASSERT() (on ARM, the ASSERT() was even
already there!)

Suggested-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Dario Faggioli <dfaggioli@suse.com>
Acked-by: Julien Grall <julien.grall@arm.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Andrii Anisov <andrii_anisov@epam.com>