[Bug tree-optimization/91882] boolean XOR tautology missed optimisation

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91882

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This is a job for reassociation pass really.
coming into that pass we have:
  _1 = a_3(D) | b_4(D);
  if (_1 != 0)
    goto <bb 3>; [50.00%]
  else
    goto <bb 4>; [50.00%]

  <bb 3> [local count: 536870913]:
  _5 = a_3(D) & b_4(D);
  if (_5 != 0)
    goto <bb 4>; [50.00%]
  else
    goto <bb 5>; [50.00%]

  <bb 4> [local count: 805306369]:

  <bb 5> [local count: 1073741824]:
  # iftmp.0_2 = PHI <1(3), 0(4)>

We should be able to transform this into:
  _6 = a_3(D) ^ b_4(D);
  if (_6 != 0)
    goto <bb 3>; [50.00%]
  else
    goto <bb 4>; [50.00%]

  <bb 3> [local count: 536870913]:

  if (0 != 0)
    goto <bb 4>; [50.00%]
  else
    goto <bb 5>; [50.00%]

  <bb 4> [local count: 805306369]:

  <bb 5> [local count: 1073741824]:
  # iftmp.0_2 = PHI <1(3), 0(4)>

then cfg cleanup should do:
  _6 = a_3(D) ^ b_4(D);
  if (_6 != 0)
    goto <bb 5>; [50.00%]
  else
    goto <bb 4>; [50.00%]

  <bb 4> [local count: 805306369]:

  <bb 5> [local count: 1073741824]:
  # iftmp.0_2 = PHI <1(1), 0(4)>

And then PHI-OPT should transform it into:
_6 = a_3(D) ^ b_4(D);
iftmp.0_2 = _6;