[turbofan] Build s/NULL/nullptr/g and CHECK(x != nullptr) to CHECK_NOT_NULL(x).

src/compiler/ppc/instruction-selector-ppc.cc

Index: src/compiler/ppc/instruction-selector-ppc.cc
diff --git a/src/compiler/ppc/instruction-selector-ppc.cc b/src/compiler/ppc/instruction-selector-ppc.cc
index 93ad4575564cf24cf3e202245726836c40d15425..f6ebbdf5d6ab249a47cc1cc5469b0a139643f8b5 100644
--- a/src/compiler/ppc/instruction-selector-ppc.cc
+++ b/src/compiler/ppc/instruction-selector-ppc.cc
@@ -1431,12 +1431,12 @@ void VisitWordCompareZero(InstructionSelector* selector, Node* user,
         if (ProjectionIndexOf(value->op()) == 1u) {
           // We cannot combine the <Operation>WithOverflow with this branch
           // unless the 0th projection (the use of the actual value of the
-          // <Operation> is either NULL, which means there's no use of the
+          // <Operation> is either nullptr, which means there's no use of the
           // actual value, or was already defined, which means it is scheduled
           // *AFTER* this branch).
           Node* const node = value->InputAt(0);
           Node* const result = NodeProperties::FindProjection(node, 0);
-          if (result == NULL || selector->IsDefined(result)) {
+          if (result == nullptr || selector->IsDefined(result)) {
             switch (node->opcode()) {
               case IrOpcode::kInt32AddWithOverflow:
                 cont->OverwriteAndNegateIfEqual(kOverflow);