x86: use only a single branch for upcall-pending exit path checks

x86: use only a single branch for upcall-pending exit path checks

This utilizes the fact that the two bytes of interest are adjacent to
one another and that the resulting 16-bit values of interest are within
a contiguous range of numbers.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>