Test CMPXCHG-FAIL

X86_64 CMPXCHG-FAIL
{
int x;
int y;
0:rax=2;
0:rbx=1;
}

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