tree-optimization/109331 - make sure to clean up the CFG after forwprop

When forwprop discovers unreachable code or makes decisions based
on unreachable edges make sure to cleanup the CFG since otherwise
SSA form can become invalid.

	PR tree-optimization/109331
	* tree-ssa-forwprop.cc (pass_forwprop::execute): When we
	discover a taken edge make sure to cleanup the CFG.

	* gcc.dg/torture/pr109331.c: New testcase.

gcc/testsuite/gcc.dg/torture/pr109331.c

@@ -0,0 +1,16 @@
+/* { dg-do compile } */
+
+char *ustol_dpp;
+void ustol(int flags)
+{
+  char *s;
+  if (s)
+    flags |= 3;
+  switch (flags & 3)
+  case 3:
+    while (*s)
+    case '+':
+      ++s;
+  if (flags)
+    ustol_dpp = s;
+}

gcc/tree-ssa-forwprop.cc

@@ -4046,7 +4046,11 @@ pass_forwprop::execute (function *fun)
 
       /* Mark outgoing exectuable edges.  */
       if (edge e = find_taken_edge (bb, NULL))
-	e->flags |= EDGE_EXECUTABLE;
+	{
+	  e->flags |= EDGE_EXECUTABLE;
+	  if (EDGE_COUNT (bb->succs) > 1)
+	    cfg_changed = true;
+	}
       else
 	{
 	  FOR_EACH_EDGE (e, ei, bb->succs)