target-i386: Fix mulx for identical target regs

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: Toni Nedialkov <farmdve@gmail.com>
Reported-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
