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

src/compiler/pipeline-statistics.h

Index: src/compiler/pipeline-statistics.h
diff --git a/src/compiler/pipeline-statistics.h b/src/compiler/pipeline-statistics.h
index 988327d1bb86d74209ab370f98795e45cda8d362..2b6563da403c4e3b40021196e642dedaaaf621d5 100644
--- a/src/compiler/pipeline-statistics.h
+++ b/src/compiler/pipeline-statistics.h
@@ -76,10 +76,10 @@ class PhaseScope {
  public:
   PhaseScope(PipelineStatistics* pipeline_stats, const char* name)
       : pipeline_stats_(pipeline_stats) {
-    if (pipeline_stats_ != NULL) pipeline_stats_->BeginPhase(name);
+    if (pipeline_stats_ != nullptr) pipeline_stats_->BeginPhase(name);
   }
   ~PhaseScope() {
-    if (pipeline_stats_ != NULL) pipeline_stats_->EndPhase();
+    if (pipeline_stats_ != nullptr) pipeline_stats_->EndPhase();
   }
 
  private: