[Bug middle-end/85989] [6/7/8/9 Regression] Incorrect result for example involving unary minus in a loop

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85989

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
  for (b_P5 = 0; b_P5 < 8; b_P5++) {
    P26 = P29;
    P29 = -(int)*P5;
  }

  for (b_P5 = 0; b_P5 < 3; b_P5++) {
    for (P29 = 0; P29 < 9; P29++) {
      i0 = P26;
      if (P26 < 0) {
        i0 = 0;

so the sign of P29 does matter since it flows backwards into P26 -- possibly
exactly the issue that causes the bug.  The PHIs look like

  <bb 5> [local count: 286363364]:
  _3 = *P5_32(D);
  _4 = (int) _3;
  P29_36 = -_4;
  b_P5_37 = b_P5_20 + 1;

  <bb 6> [local count: 322154758]:
  # b_P5_20 = PHI <b_P5_37(5), 0(16)>
  # P26_22 = PHI <P29_23(5), 10(16)>
  # P29_23 = PHI <P29_36(5), 10(16)>
  if (b_P5_20 <= 7)
    goto <bb 5>; [88.89%]
  else
    goto <bb 14>; [11.11%]

and you can see that parallel assignment to P29_23 and to P23_22 from P29_23
while backprop seems to evaluate PHIs in isolation, eventually losing
that P26_22 = P29_23 copy?  This looks hard to fix btw...  process_block
needs to be refactored to process all vars from PHI defs in parallel.

Richard?