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

src/compiler/register-allocator.cc

Index: src/compiler/register-allocator.cc
diff --git a/src/compiler/register-allocator.cc b/src/compiler/register-allocator.cc
index 80fdb135e38a33d5b2e8de0ee5c7d61689cd61e7..23da999119118176e606992d4e8f41b9be599414 100644
--- a/src/compiler/register-allocator.cc
+++ b/src/compiler/register-allocator.cc
@@ -287,10 +287,10 @@ void LiveRange::VerifyPositions() const {
   for (UsePosition* pos = first_pos_; pos != nullptr; pos = pos->next()) {
     CHECK(Start() <= pos->pos());
     CHECK(pos->pos() <= End());
-    CHECK(interval != nullptr);
+    CHECK_NOT_NULL(interval);
     while (!interval->Contains(pos->pos()) && interval->end() != pos->pos()) {
       interval = interval->next();
-      CHECK(interval != nullptr);
+      CHECK_NOT_NULL(interval);
     }
   }
 }