[Bug tree-optimization/66652] try_transform_to_exit_first_loop_alt generates incorrect loop

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66652

--- Comment #2 from vries at gcc dot gnu.org ---
Created attachment 35865
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35865&action=edit
follow-up patch for demonstrator patch

This patch in addition allows transform_to_exit_first_loop_alt for unsigned
loop counters. But it's not generic enough to also handle the fortran testcases
in https://gcc.gnu.org/ml/gcc-patches/2015-06/msg01234.html .

For parloops-exit-first-loop-alt.f95, we have:
...
foo (integer(kind=4)D.8 & restrict nrD.3381)
{
  integer(kind=4) ii;
  integer(kind=4) _6;
  integer(kind=4) _7;
  integer(kind=8) _8;
  integer(kind=4) _9;
  integer(kind=4) _10;
  integer(kind=4) _11;
  integer(kind=4) _12;
  integer(kind=4) _13;
  integer(kind=8) _16;
  unsigned int _19;
  integer(kind=4) _20;
  integer(kind=4) _22;
  integer(kind=4) _23;
  integer(kind=4) _24;
  signed int ivtmp_27;
  signed int ivtmp_28;

  <bb 2>:
  _6 = *nr_5(D);
  _7 = _6 + -1;
  if (_7 >= 0)
    goto <bb 4>;
  else
    goto <bb 3>;

  <bb 3>:
  return;

  <bb 4>:
  _20 = _6 + -1;
  _19 = (unsigned int) _20;

  <bb 10>:
  if (_19 > 199)
    goto <bb 11>;
  else
    goto <bb 12>;

  <bb 11>:

  <bb 5>:
  # ivtmp_27 = PHI <0(11), ivtmp_28(7)>
  ii_1 = ivtmp_27;
  _8 = (integer(kind=8)) ii_1;
  _9 = __BLNK__.a[_8];
  _11 = __BLNK__.b[_8];
  _12 = _9 + _11;
  _13 = _12 + 25;
  __BLNK__.c[_8] = _13;
  ii_15 = ii_1 + 1;
  if (ivtmp_27 < _19)
    goto <bb 7>;
  else
    goto <bb 6>;

  <bb 6>:
  goto <bb 3>;

  <bb 7>:
  ivtmp_28 = ivtmp_27 + 1;
  goto <bb 5>;

  <bb 12>:

  <bb 8>:
  # ii_18 = PHI <0(12), ii_26(9)>
  _16 = (integer(kind=8)) ii_18;
  _10 = __BLNK__.a[_16];
  _23 = __BLNK__.b[_16];
  _22 = _10 + _23;
  _24 = _22 + 25;
  __BLNK__.c[_16] = _24;
  ii_26 = ii_18 + 1;
  if (ii_18 == _7)
    goto <bb 6>;
  else
    goto <bb 9>;

  <bb 9>:
  goto <bb 8>;

}
...

the nit is _19:
...
  _20 = _6 + -1;
  _19 = (unsigned int) _20;
...