The Intel specification clearly indicates that the low part
of the result is written first and the high part of the result
is written second; thus if ModRM:reg and VEX.vvvv are identical,
the final result should be the high part of the result.

At present, TCG may either produce incorrect results or crash
with --enable-checking.
Reported-by: NToni Nedialkov <farmdve@gmail.com>
Reported-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NRichard Henderson <rth@twiddle.net>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>