Fix a llvm-tier getiter() bug

If we find a type that
- we think we can reason about statically
- does not have an __iter__ but can be iterated via __getitem__

we previously just bailed saying that we know it doesn't have an __iter__ method
(instead of calling getiterHelper like we should).  This bug exists on master as well,
but we just don't run into it that often (especially since it's llvm-tier-only).

src/codegen/compvars.cpp

@@ -1973,8 +1973,16 @@ public:
                                const std::vector<BoxedString*>* keyword_names) override {
         ExceptionStyle exception_style = info.preferredExceptionStyle();
 
+        bool no_attribute = false;
+        bool* no_attribute_ptr = NULL;
+        if (flags.null_on_nonexistent)
+            no_attribute_ptr = &no_attribute;
+
         CompilerVariable* called_constant = tryCallattrConstant(emitter, info, var, attr, flags.cls_only, flags.argspec,
-                                                                args, keyword_names, NULL, exception_style);
+                                                                args, keyword_names, no_attribute_ptr, exception_style);
+
+        if (no_attribute)
+            return new ConcreteCompilerVariable(UNKNOWN, getNullPtr(g.llvm_value_type_ptr));
 
         if (called_constant)
             return called_constant;

src/runtime/iterobject.h

@@ -41,7 +41,7 @@ public:
 
     static void dealloc(BoxedSeqIter* o) noexcept {
         PyObject_GC_UnTrack(o);
-        Py_DECREF(o->b);
+        Py_XDECREF(o->b);
         Py_XDECREF(o->next);
         o->cls->tp_free(o);
     }