[Bug tree-optimization/62217] DOM confuses complete unrolling which in turn causes VRP to warn

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62217

--- Comment #3 from Kirill Yukhin <kyukhin at gcc dot gnu.org> ---
As long as I understand `remove_exits_and_undefined_stmts'
iterate loop boundaries set marking `unreachable' stmts w/
impossible bounds.

For the example we have:
- for true edge
  basic block 6, loop depth 1
   pred:       5
  g_x[b_6(D)] = *x1_7(D);
  goto <bb 8>;
   succ:       8
- for false edge
  basic block 7, loop depth 1
   pred:       5
  g_x[i_14] = *x2_9(D);
   succ:       8

I suspect, that the problem is that `b' was propagated along
`true' edge and replaced use of `i'.
This removed reference to g_x from boundaries analysis for that edge:
no IV is used there explicitly, only implicitly as b == i.

Hence this stmt didn't hit boundaries set of the loop and
wasn't marked as unreachable.

BTW: this code survive rest of optimizations:
        movl    (%edx), %edx
        cmpl    $4, %eax
        movl    %edx, g_x+12
        jle     .L1
        movl    (%ebx), %eax
        movl    %eax, g_x+16 ;; REDUNDANT

Looks like not simple warning, but also unnecessary code was
generated.