[Bug middle-end/93041] GCC 10.0 with '-Os', '-O2' and '-O3' compiled program crashes when it shouldn't

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93041

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
So what is happening is (infinite) loops can be removed if they have no side
effects. 
So we start with:
  <bb 2> :
label1:
  if (p_3(D) != 0B)
    goto <bb 3>; [INV]
  else
    goto <bb 4>; [INV]

  <bb 3> :
  // predicted unlikely by goto predictor.
  goto <bb 2>; [INV]

  <bb 4> :
  _1 = *p_3(D);
  _2 = *_1;
  g3 = _2;
  return;


And then we prop 0 into p_3 and then we remove the infinite loop.
So we run into undefined behavior from dereferencing a null pointer.

I think this code is undefined due to forward progress is required by the
standards.