Allow stat += 0 without activating the stat.

For me, this is a nice convenience. We generally want grep to match
stats output only when the event has occurred.

llvm-svn: 141574

include/llvm/ADT/Statistic.h

@@ -84,11 +84,13 @@ public:
   }
 
   const Statistic &operator+=(const unsigned &V) {
+    if (!V) return *this;
     sys::AtomicAdd(&Value, V);
     return init();
   }
 
   const Statistic &operator-=(const unsigned &V) {
+    if (!V) return *this;
     sys::AtomicAdd(&Value, -V);
     return init();
   }