DebugInfo: Hide isScopeRef() and isTypeRef() behind NDEBUG

The copies of these in `lib/IR/DebugInfo.cpp` are apparently [1] only
used in assertions now, so hide them behind `#ifndef NDEBUG`.

[1]: http://lab.llvm.org:8011/builders/lld-x86_64-darwin13/builds/9238

llvm-svn: 233661
diff --git a/llvm/lib/IR/DebugInfo.cpp b/llvm/lib/IR/DebugInfo.cpp
index 9381a5b..aa61283 100644
--- a/llvm/lib/IR/DebugInfo.cpp
+++ b/llvm/lib/IR/DebugInfo.cpp
@@ -223,6 +223,7 @@
 bool DICompileUnit::Verify() const { return isCompileUnit(); }
 bool DIObjCProperty::Verify() const { return isObjCProperty(); }
 
+#ifndef NDEBUG
 /// \brief Check if a value can be a reference to a type.
 static bool isTypeRef(const Metadata *MD) {
   if (!MD)
@@ -241,7 +242,6 @@
   return isa<MDScope>(MD);
 }
 
-#ifndef NDEBUG
 /// \brief Check if a value can be a DescriptorRef.
 static bool isDescriptorRef(const Metadata *MD) {
   if (!MD)