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/optimizing/gvn.cc b/compiler/optimizing/gvn.cc
index 74848d5..708733e 100644
--- a/compiler/optimizing/gvn.cc
+++ b/compiler/optimizing/gvn.cc
@@ -55,7 +55,7 @@
         buckets_owned_(allocator, num_buckets_, false),
         num_entries_(to_copy.num_entries_) {
     // ArenaAllocator returns zeroed memory, so entries of buckets_ and
-    // buckets_owned_ are initialized to nullptr and false, respectively.
+    // buckets_owned_ are initialized to null and false, respectively.
     DCHECK(IsPowerOfTwo(num_buckets_));
     if (num_buckets_ == to_copy.num_buckets_) {
       // Hash table remains the same size. We copy the bucket pointers and leave