Replace NULL with nullptr

Also fixed some lines that were too long, and a few other minor
details.

Change-Id: I6efba5fb6e03eb5d0a300fddb2a75bf8e2f175cb
diff --git a/compiler/utils/dedupe_set.h b/compiler/utils/dedupe_set.h
index b062a2a..a9a5781 100644
--- a/compiler/utils/dedupe_set.h
+++ b/compiler/utils/dedupe_set.h
@@ -40,8 +40,8 @@
   struct HashedKey {
     StoreKey* store_ptr;
     union {
-      HashType store_hash;        // Valid if store_ptr != nullptr.
-      const HashedInKey* in_key;  // Valid if store_ptr == nullptr.
+      HashType store_hash;        // Valid if store_ptr != null.
+      const HashedInKey* in_key;  // Valid if store_ptr == null.
     };
   };