[Bug tree-optimization/66163] [6 Regression] Not working Firefox built with LTO

[law at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66163

--- Comment #9 from Jeffrey A. Law <law at redhat dot com> ---
We don't thread because we don't lower UBSAN_NULL to actual conditionals until
the end of the gimple/ssa pipeline.  ie, in DOM2 we have:

int test::foo(int&) (struct test * const this, int & b)
{
  int _5;
  int _6;

;;   basic block 2, loop depth 0, count 0, freq 10000, maybe hot
;;    prev block 0, next block 3, flags: (NEW, REACHABLE)
;;    pred:       ENTRY [100.0%]  (FALLTHRU,EXECUTABLE)
  if (this_2(D) != 0B)
    goto <bb 3>;
  else
    goto <bb 4>;
;;    succ:       3 [78.3%]  (TRUE_VALUE,EXECUTABLE)
;;                4 [21.6%]  (FALSE_VALUE,EXECUTABLE)

;;   basic block 3, loop depth 0, count 0, freq 7835, maybe hot
;;    prev block 2, next block 4, flags: (NEW, REACHABLE)
;;    pred:       2 [78.3%]  (TRUE_VALUE,EXECUTABLE)
  UBSAN_NULL (this_2(D), 3B, 0);
  _5 = this_2(D)->a;
;;    succ:       4 [100.0%]  (FALLTHRU,EXECUTABLE)

;;   basic block 4, loop depth 0, count 0, freq 10000, maybe hot
;;    prev block 3, next block 1, flags: (NEW, REACHABLE)
;;    pred:       2 [21.6%]  (FALSE_VALUE,EXECUTABLE)
;;                3 [100.0%]  (FALLTHRU,EXECUTABLE)
  # _6 = PHI <1(2), _5(3)>
  return _6;
;;    succ:       EXIT [100.0%]  (EXECUTABLE)

}