[Bug tree-optimization/99395] s116 benchmark of TSVC is vectorized by clang and not by gcc

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99395

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
So in the BB SLP attempt from loop vectorization (or in the BB SLP pass with
-fno-predictive-commoning) we get confused during DR group building because
of a duplicate access and fixup splitting the candidates at odd points.

For the reduced testcase we see

  <bb 3> [local count: 1063004409]:
  # i_16 = PHI <_5(5), 0(2)>
  # ivtmp_18 = PHI <ivtmp_15(5), 511(2)>
  _1 = i_16 + 1;
  _2 = a[_1];
  _3 = a[i_16];
  _4 = _2 * _3;
  a[i_16] = _4;
  _5 = i_16 + 2;
  _6 = a[_5];
  _7 = a[_1];
  _8 = _6 * _7;
  a[_1] = _8;
  ivtmp_15 = ivtmp_18 - 1;
  if (ivtmp_15 != 0)
    goto <bb 5>; [99.00%]
  else
    goto <bb 4>; [1.00%]

so a[_1] is loaded twice because CSE doesn't figure that a[i_16] cannot alias
it.  That causes us to split the load group.