SingleLinkedListIterator::operator++(int) shouldn't return a reference

The returned reference is to a local object.  Instead, make a copy.

Found by PVS-Studio.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@285603 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/LiveInterval.h

@@ -672,7 +672,7 @@ namespace llvm {
         P = P->Next;
         return *this;
       }
-      SingleLinkedListIterator<T> &operator++(int) {
+      SingleLinkedListIterator<T> operator++(int) {
         SingleLinkedListIterator res = *this;
         ++*this;
         return res;