Always perform a sign-extending load.  In the extremely unlikely
case that we've used an 0x66 prefix, the extension to 64-bits is
unnecessary but not wrong; the store will still examine only 16 bits.
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NRichard Henderson <rth@twiddle.net>