[turbofan] Build s/NULL/nullptr/g and CHECK(x != nullptr) to CHECK_NOT_NULL(x).

src/compiler/bytecode-graph-builder.h

Index: src/compiler/bytecode-graph-builder.h
diff --git a/src/compiler/bytecode-graph-builder.h b/src/compiler/bytecode-graph-builder.h
index 9c22251e0f478a248cfc227d82d14e059937d43a..d5d0fc95ae79fd10f81629bda49d6a93a2867b1b 100644
--- a/src/compiler/bytecode-graph-builder.h
+++ b/src/compiler/bytecode-graph-builder.h
@@ -65,7 +65,7 @@ class BytecodeGraphBuilder {
 
   // Node creation helpers
   Node* NewNode(const Operator* op, bool incomplete = false) {
-    return MakeNode(op, 0, static_cast<Node**>(NULL), incomplete);
+    return MakeNode(op, 0, static_cast<Node**>(nullptr), incomplete);
   }
 
   Node* NewNode(const Operator* op, Node* n1) {