[Bug tree-optimization/126388] BB SLP promoted external node sources not considered live

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126388

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
It correctly works for the following so it might be special for extern permute
nodes, possibly with some clever two-operator case that's not supported, not
possible on x86 at least.

void foo (long *p, long *q, long *r, long s)
{
  long tem0 = p[0];
  long tem1 = p[1];
  tem0 = tem0 + 1;
  tem1 = tem1 + 2;
  q[0] = tem0 / s;
  q[1] = tem1 / s;
  r[0] = tem0;
  r[1] = tem1;
}