target-i386: use inverted setcond when computing NS or NZ

target-i386: use inverted setcond when computing NS or NZ

Make gen_compute_eflags_z and gen_compute_eflags_s able to compute the
inverted condition, and use this in gen_setcc_slow_T0.  We cannot do it
yet in gen_compute_eflags_c, but prepare the code for it anyway.  It is
not worthwhile for PF, as usual.

shr+and+xor could be replaced by and+setcond.  I'm not doing it yet.

Reviewed-by: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>