small speedup in the case where a smallvector is default ctor'd from

an empty vector.  This speeds up llc slightly.

llvm-svn: 40983

llvm/include/llvm/ADT/SmallVector.h

@@ -455,7 +455,8 @@ public:
   }
   
   SmallVector(const SmallVector &RHS) : SmallVectorImpl<T>(NumTsAvailable) {
-    operator=(RHS);
+    if (!RHS.empty())
+      operator=(RHS);
   }
   
   const SmallVector &operator=(const SmallVector &RHS) {