Minor fixes in ASViewController:

- If its node isn't provided by users, don't replace the view controller's view with the node's view because it might be loaded from a nib.
- Init a vanilla ASDisplayNode if a node isn't provided.

AsyncDisplayKit/ASViewController.mm

@@ -24,6 +24,7 @@
 
 @implementation ASViewController
 {
+  BOOL _nodeProvided;
   BOOL _ensureDisplayed;
   BOOL _automaticallyAdjustRangeModeBasedOnViewEvents;
   BOOL _parentManagesVisibilityDepth;
@@ -38,9 +39,7 @@ - (instancetype)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibB
     return nil;
   }
 
-  _node = [[ASDisplayNode alloc] initWithViewBlock:^UIView * _Nonnull{
-    return [[UIView alloc] init];
-  }];
+  _node = [[ASDisplayNode alloc] init];
   [self _initializeInstance];
 
   return self;
@@ -52,9 +51,7 @@ - (instancetype)initWithCoder:(NSCoder *)aDecoder
     return nil;
   }
 
-  _node = [[ASDisplayNode alloc] initWithViewBlock:^UIView * _Nonnull{
-    return [[UIView alloc] init];
-  }];
+  _node = [[ASDisplayNode alloc] init];
   [self _initializeInstance];
 
   return self;
@@ -66,6 +63,7 @@ - (instancetype)initWithNode:(ASDisplayNode *)node
     return nil;
   }
 
+  _nodeProvided = YES;
   _node = node;
   [self _initializeInstance];
 
@@ -104,14 +102,16 @@ - (void)dealloc
 
 - (void)loadView
 {
-  ASDisplayNodeAssertTrue(!_node.layerBacked);
-
   // Apple applies a frame and autoresizing masks we need.  Allocating a view is not
   // nearly as expensive as adding and removing it from a hierarchy, and fortunately
   // we can avoid that here.  Enabling layerBacking on a single node in the hierarchy
   // will have a greater performance benefit than the impact of this transient view.
   [super loadView];
 
+  if (!_nodeProvided) { return; }
+
+  ASDisplayNodeAssertTrue(!_node.layerBacked);
+
   UIView *view = self.view;
   CGRect frame = view.frame;
   UIViewAutoresizing autoresizingMask = view.autoresizingMask;