Hi,
For the following test-case (slightly reduced from PR)
int a, b, c;

int g() {
  char i = 0;
  for (c = 0; c <= 8; c++)
    --i;

  while (b) {
    _Bool f = i <= 0;
    a = (a == 0) ? 0 : f / a;
  }
}

The compiler segfaults with -O1 -march=armv8.2-a+sve in ifcvt_local_dce.

IIUC, the issue here is that tree-if-conv.c:predicate_rhs_code
processes the following statement:
iftmp.2_7 = a_lsm.10_11 != 0 ? iftmp.2_13 : 0;
and records <iftmp.2_7, iftmp.2_13> mapping.

However RPO VN eliminates iftmp.2_13:
Removing dead stmt iftmp.2_13 = .COND_DIV (_29, _4, a_lsm.10_11, 0);

and we end up replacing iftmp.2_7 with a dead ssa_name in ifcvt_local_dce:
FOR_EACH_VEC_ELT (redundant_ssa_names, i, name_pair)
    replace_uses_by (name_pair->first, name_pair->second);
  redundant_ssa_names.release ();

resulting in incorrect IR, and segfault down the line.

To avoid clashing of RPO VN with redunant_ssa_names, the patch simply moves
ifcvt_local_dce before do_rpo_vn, which avoids the segfault.
Does that look OK ?
(Altho I guess, doing DCE after VN is better in principle)

Thanks,
Prathamesh