sim: bfin: update VIT_MAX behavior to match hardware when Acc.X bits are set

sim: bfin: update VIT_MAX behavior to match hardware when Acc.X bits are set

The Blackfin PRM says that the top 8 bits of the accumulator must be
cleared when using the VIT_MAX insn, so the sim has followed this spec.
Matching the hardware behavior though when the high bits are not cleared
is easy to do and doesn't break existing behavior, so go for it.

Signed-off-by: Robin Getz <robin.getz@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>