[Bug tree-optimization/102139] [11/12 Regression] -O3 miscompile due to slp-vectorize on strict align target

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102139

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #3)
> There is no PHI for a for FRE to merge even. And there is no alignment
> information on the pointers assignments either.

The only thing FRE does for this testcase which SLP might make a huge mistake
on is prop a_11(D) into the MEM.
From:
  # i_24 = PHI <0(2)>
  _22 = i_24 * 8;
  _25 = a_11(D) + _22;
  MEM[(vector(4) short unsigned int *)_25] = { 0, 0, 0, 0 };
Into:
  MEM[(vector(4) short unsigned int *)a_11(D)] = { 0, 0, 0, 0 };

But SLP seems like does not take into account the two sides of the branches are
unrelated and still uses the alignment of the first for the second.

/app/example.cpp:6:10: note:  recording new base alignment for a_11(D)
  alignment:    2
  misalignment: 0
  based on:     *a_11(D) = 0;
/app/example.cpp:6:10: note:  recording new base alignment for a_11(D)
  alignment:    8
  misalignment: 0
  based on:     MEM[(vector(4) short unsigned int *)a_11(D)] = { 0, 0, 0, 0 };