phiopt: Fix phiopt ICE on vops [PR102989]

I've ran into ICE on gcc.dg/torture/bitint-42.c with -O1 or -Os
when enabling expensive tests, and unfortunately I can't reproduce without
_BitInt.  The IL before phiopt3 has:
  <bb 87> [local count: 203190070]:
  # .MEM_428 = VDEF <.MEM_367>
  bitint.159 = VIEW_CONVERT_EXPR<unsigned long[8]>(*.LC3);
  goto <bb 89>; [100.00%]

  <bb 88> [local count: 203190070]:
  # .MEM_427 = VDEF <.MEM_367>
  bitint.159 = VIEW_CONVERT_EXPR<unsigned long[8]>(*.LC4);

  <bb 89> [local count: 406380139]:
  # .MEM_368 = PHI <.MEM_428(87), .MEM_427(88)>
  # VUSE <.MEM_368>
  _123 = VIEW_CONVERT_EXPR<unsigned long[8]>(r495[i_107].D.2780)[0];
and factor_out_conditional_operation is called on the vop PHI, it
sees it has exactly two operands and defining statements of both
PHI arguments are converts (VCEs in this case), so it thinks it is
a good idea to try to optimize that and while doing that it constructs
void type SSA_NAMEs and the like.

2023-08-10  Jakub Jelinek  <jakub@redhat.com>

	PR c/102989
	* tree-ssa-phiopt.cc (single_non_singleton_phi_for_edges): Never
	return virtual phis and return NULL if there is a virtual phi
	where the arguments from E0 and E1 edges aren't equal.