I am not sure why I made it use T1. It is a bit more symmetric with
respect to X86_ENTRYwr (which uses T0 for the "w"ritten operand
and T1 for the "r"ead operand), but it is also less flexible because it
does not let you apply zextT0/sextT0.
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>