Test CMPXCHG-UNLOCKED

X86_64 CMPXCHG-UNLOCKED
{
int x;
int y;
0:rbx=2;
}

 P0                     | P1             ;
 movl $1,(x)            | movl $1,(y)    ;
 cmpxchg (z),%ebx       | mfence         ;
 movl (y),%ecx          | movl (x),%ecx  ;
exists (0:rcx=0 /\ 1:rcx=0)