[Bug tree-optimization/95923] Failure to optimize bool checks into and

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95923

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
After r14-1597-g64d90d06d2db, we now have:
  _8 = a_6(D) | b_7(D);
  _10 = a_6(D) == b_7(D);
  _1 = _8 & _10;

(a == b) & (a | b)

I am no longer working on this right now. That is a job for reassociate I
think.

Note I noticed that we don't remove some "dead" statements during phiopt3 too:
```
  if (_8 != 0)
    goto <bb 3>; [66.00%]
  else
    goto <bb 4>; [34.00%]

  <bb 3> [local count: 708669601]:
  _1 = ~a_6(D);
  _2 = _1 & b_7(D);
  _10 = a_6(D) == b_7(D);

  <bb 4> [local count: 1073741824]:
  # _5 = PHI <_10(3), 0(2)>
```

I will fix that before unassigning. The problem there is:
We had:
  _1 = ~a_6(D);
  _2 = _1 & b_7(D);
  if (_2 != 0)

But we don't mark _2 as possible unused even though match will produce without
it:
Folded into the sequence:
_3 = ~b_7(D);
_4 = _3 | a_6(D);
_11 = a_6(D) ^ b_7(D);
_10 = a_6(D) == b_7(D);
statement un-sinked:
_12 = _1 | b_7(D);