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

src/compiler/ast-graph-builder.h

Index: src/compiler/ast-graph-builder.h
diff --git a/src/compiler/ast-graph-builder.h b/src/compiler/ast-graph-builder.h
index 96c1a911565ce0c3fdcf427f886befb55cd00aa2..3b6302d3ddcf86bd719df0d910e28314ddc3e684 100644
--- a/src/compiler/ast-graph-builder.h
+++ b/src/compiler/ast-graph-builder.h
@@ -169,7 +169,7 @@ class AstGraphBuilder : public AstVisitor {
 
   // 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) {
@@ -365,7 +365,7 @@ class AstGraphBuilder : public AstVisitor {
 
   // ===========================================================================
   // The following build methods have the same contract as the above ones, but
-  // they can also return {NULL} to indicate that no fragment was built. Note
+  // they can also return {nullptr} to indicate that no fragment was built. Note
   // that these are optimizations, disabling any of them should still produce
   // correct graphs.