[Bug tree-optimization/69109] [6 Regression] missing phi argument ICE in transform_to_exit_first_loop_alt with -O2 -funswitch-loops -ftree-parallelize-loops=2

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69109

--- Comment #2 from vries at gcc dot gnu.org ---
-funswitch-loops introduces degenerate phis in a loop latch (bb 9):
...
;;   basic block 9, loop depth 1, count 0, freq 4900, maybe hot
;;    prev block 8, next block 10, flags: (NEW, REACHABLE)
;;    pred:       10 [99.0%]  (TRUE_VALUE,EXECUTABLE)
  # j_1 = PHI <j_11(10)>
  # .MEM_2 = PHI <.MEM_10(10)>
;;    succ:       10 [100.0%]  (FALLTHRU,EXECUTABLE)

;;   basic block 10, loop depth 1, count 0, freq 4950, maybe hot
;;    prev block 9, next block 11, flags: (NEW, REACHABLE)
;;    pred:       9 [100.0%]  (FALLTHRU,EXECUTABLE)
;;                3 [100.0%]  (FALLTHRU)
  # RANGE [0, 256] NONZERO 511
  # j_17 = PHI <j_1(9), 0(3)>
  # .MEM_18 = PHI <.MEM_2(9), .MEM_4(D)(3)>
  # RANGE [0, 255] NONZERO 255
  _7 = (sizetype) j_17;
  # PT = nonlocal
  _9 = fastmap_8(D) + _7;
  # .MEM_10 = VDEF <.MEM_18>
  *_9 = 1;
  # RANGE [1, 256] NONZERO 511
  j_11 = j_17 + 1;
  if (j_11 != 256)
    goto <bb 9>;
  else
    goto <bb 11>;
;;    succ:       9 [99.0%]  (TRUE_VALUE,EXECUTABLE)
;;                11 [1.0%]  (FALSE_VALUE,EXECUTABLE)
...

The degenerate PHI is introduced by cleanup_tree_cfg triggered by
TODO_cleanup_cfg.

The j_1 PHI is removed by pass_copy_prop before parloops, but the .MEM_2 PHI
not.

Adding pass_phi_only_cprop after pass_tree_unswitch gets rid of both (even if
it is only verbose about getting rid of the j_1 PHI).

Anyway, transform_to_exit_first_loop_alt asserts because it doesn't know how to
handle a phi in the latch.