Revert "Cannot use nullptr in previous commit. Replaced with 0"

This reverts commit 8938b17c.

llvm/lib/Transforms/IPO/ArgumentPromotion.cpp

@@ -130,7 +130,7 @@ CallGraphNode *ArgPromotion::PromoteArguments(CallGraphNode *CGN) {
   // parameters. Frontends encode that classification at the call site in the
   // IR, while in the callee the classification is determined dynamically
   // on the number of registers consumed so far.
-  if (F->isVarArg()) return 0;
+  if (F->isVarArg()) return nullptr;
 
   // First check: see if there are any pointer arguments!  If not, quick exit.
   SmallVector<std::pair<Argument*, unsigned>, 16> PointerArgs;