Conditionalize function only used in an assert to address -Wunused-function

lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp

@@ -3606,15 +3606,19 @@ LDVSSABlock *LDVSSABlockIterator::operator*() {
   return Updater.getSSALDVBlock(*PredIt);
 }
 
-} // namespace
-
-namespace llvm {
+#ifndef NDEBUG
 
 raw_ostream &operator<<(raw_ostream &out, const LDVSSAPhi &PHI) {
   out << "SSALDVPHI " << PHI.PHIValNum;
   return out;
 }
 
+#endif
+
+} // namespace
+
+namespace llvm {
+
 /// Template specialization to give SSAUpdater access to CFG and value
 /// information. SSAUpdater calls methods in these traits, passing in the
 /// LDVSSAUpdater object, to learn about blocks and the values they define.