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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206243 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/InstCount.cpp

@@ -47,7 +47,7 @@ namespace {
 
     void visitInstruction(Instruction &I) {
       errs() << "Instruction Count does not know about " << I;
-      llvm_unreachable(0);
+      llvm_unreachable(nullptr);
     }
   public:
     static char ID; // Pass identification, replacement for typeid