[C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr.

llvm-svn: 206243
diff --git a/llvm/lib/Analysis/IntervalPartition.cpp b/llvm/lib/Analysis/IntervalPartition.cpp
index 2e259b1..a0583e8 100644
--- a/llvm/lib/Analysis/IntervalPartition.cpp
+++ b/llvm/lib/Analysis/IntervalPartition.cpp
@@ -29,7 +29,7 @@
     delete Intervals[i];
   IntervalMap.clear();
   Intervals.clear();
-  RootInterval = 0;
+  RootInterval = nullptr;
 }
 
 void IntervalPartition::print(raw_ostream &O, const Module*) const {