tree-optimization/112939 - VN PHI visiting and -ftrivial-auto-var-init

The following builds upon the last fix, making sure we only value-number
to visited (un-)defs, otherwise prefer .VN_TOP.

	PR tree-optimization/112939
	* tree-ssa-sccvn.cc (visit_phi): When all args are undefined
	make sure we end up with a value that was visited, otherwise
	fall back to .VN_TOP.

	* gcc.dg/pr112939.c: New testcase.

gcc/testsuite/gcc.dg/pr112939.c

@@ -0,0 +1,23 @@
+/* { dg-do compile } */
+/* { dg-options "-O -ftrivial-auto-var-init=zero" } */
+
+int i;
+
+void f (void)
+{
+  for (;;)
+  {
+    if (0)
+      for (;;)
+      {
+        int *a;
+        int *b = a;
+
+ l1:
+        *b = (*b != 0) ? 0 : 2;
+      }
+
+    if (i != 0)
+      goto l1;
+  }
+}

gcc/tree-ssa-sccvn.cc

@@ -5946,6 +5946,8 @@ visit_phi (gimple *phi, bool *inserted, bool backedges_varying_p)
 	if (TREE_CODE (def) == SSA_NAME)
 	  {
 	    tree val = SSA_VAL (def, &visited);
+	    if (SSA_NAME_IS_DEFAULT_DEF (def))
+	      visited = true;
 	    if (!backedges_varying_p || !(e->flags & EDGE_DFS_BACK))
 	      def = val;
 	    if (e->flags & EDGE_DFS_BACK)
@@ -6091,7 +6093,7 @@ visit_phi (gimple *phi, bool *inserted, bool backedges_varying_p)
   /* If we saw only undefined values and VN_TOP use one of the
      undefined values.  */
   else if (sameval == VN_TOP)
-    result = seen_undef ? seen_undef : sameval;
+    result = (seen_undef && seen_undef_visited) ? seen_undef : sameval;
   /* First see if it is equivalent to a phi node in this block.  We prefer
      this as it allows IV elimination - see PRs 66502 and 67167.  */
   else if ((result = vn_phi_lookup (phi, backedges_varying_p)))