Don't crash lowering self-referential variable initializer.

From-SVN: r170194

gcc/go/gofrontend/gogo.cc

@@ -1139,7 +1139,8 @@ class Lower_parse_tree : public Traverse
 {
  public:
   Lower_parse_tree(Gogo* gogo, Named_object* function)
-    : Traverse(traverse_constants
+    : Traverse(traverse_variables
+	       | traverse_constants
 	       | traverse_functions
 	       | traverse_statements
 	       | traverse_expressions),
@@ -1147,6 +1148,9 @@ class Lower_parse_tree : public Traverse
   { }
 
   int
+  variable(Named_object*);
+
+  int
   constant(Named_object*, bool);
 
   int
@@ -1167,6 +1171,18 @@ class Lower_parse_tree : public Traverse
   int iota_value_;
 };
 
+// Lower variables.  We handle variables specially to break loops in
+// which a variable initialization expression refers to itself.  The
+// loop breaking is in lower_init_expression.
+
+int
+Lower_parse_tree::variable(Named_object* no)
+{
+  if (no->is_variable())
+    no->var_value()->lower_init_expression(this->gogo_, this->function_);
+  return TRAVERSE_CONTINUE;
+}
+
 // Lower constants.  We handle constants specially so that we can set
 // the right value for the predeclared constant iota.  This works in
 // conjunction with the way we lower Const_expression objects.