target-i386: Fix aflag logic for CODE64 and the 0x67 prefix

target-i386: Fix aflag logic for CODE64 and the 0x67 prefix

The code reorganization in commit 4a6fd938 broke handling of PREFIX_ADR.
While fixing this, tidy and comment the code so that it's more obvious
what's going on in setting both aflag and dflag.

The TARGET_X86_64 ifdef can be eliminated because CODE64 expands to the
constant zero when TARGET_X86_64 is undefined.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Reported-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1369855851-21400-1-git-send-email-rth@twiddle.net
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
(cherry picked from commit dec3fc9657e0682637de4d5a29d947284d01985c)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>