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

src/compiler/representation-change.cc

Index: src/compiler/representation-change.cc
diff --git a/src/compiler/representation-change.cc b/src/compiler/representation-change.cc
index df9d5033550ef11ec8e3cb2522e2879475ec4f90..bc0850f43af271f68bd2fdfd92eb143bd0ac1701 100644
--- a/src/compiler/representation-change.cc
+++ b/src/compiler/representation-change.cc
@@ -428,7 +428,7 @@ const Operator* RepresentationChanger::Int32OperatorFor(
       return machine()->Int32LessThanOrEqual();
     default:
       UNREACHABLE();
-      return NULL;
+      return nullptr;
   }
 }
 
@@ -454,7 +454,7 @@ const Operator* RepresentationChanger::Uint32OperatorFor(
       return machine()->Uint32LessThanOrEqual();
     default:
       UNREACHABLE();
-      return NULL;
+      return nullptr;
   }
 }
 
@@ -480,7 +480,7 @@ const Operator* RepresentationChanger::Float64OperatorFor(
       return machine()->Float64LessThanOrEqual();
     default:
       UNREACHABLE();
-      return NULL;
+      return nullptr;
   }
 }