Use a more correct atomic increment style.  This isn't really necessary in
this case, but it should help avoid issues in the future.

llvm-svn: 74178
diff --git a/llvm/lib/Analysis/IPA/Andersens.cpp b/llvm/lib/Analysis/IPA/Andersens.cpp
index a22a2fb..4ace049 100644
--- a/llvm/lib/Analysis/IPA/Andersens.cpp
+++ b/llvm/lib/Analysis/IPA/Andersens.cpp
@@ -285,8 +285,8 @@
 
       // Timestamp a node (used for work list prioritization)
       void Stamp() {
-        Timestamp = Counter;
-        sys::AtomicIncrement(&Counter);
+        Timestamp = sys::AtomicIncrement(&Counter);
+        --Timestamp;
       }
 
       bool isRep() const {