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

llvm-svn: 206142
diff --git a/llvm/lib/CodeGen/MachinePassRegistry.cpp b/llvm/lib/CodeGen/MachinePassRegistry.cpp
index cb204fd..3ee3e40 100644
--- a/llvm/lib/CodeGen/MachinePassRegistry.cpp
+++ b/llvm/lib/CodeGen/MachinePassRegistry.cpp
@@ -20,7 +20,7 @@
 
 /// setDefault - Set the default constructor by name.
 void MachinePassRegistry::setDefault(StringRef Name) {
-  MachinePassCtor Ctor = 0;
+  MachinePassCtor Ctor = nullptr;
   for(MachinePassRegistryNode *R = getList(); R; R = R->getNext()) {
     if (R->getName() == Name) {
       Ctor = R->getCtor();