StackColoring: Move set instead of copying. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230864 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/StackColoring.cpp

@@ -364,7 +364,7 @@ void StackColoring::calculateLocalLiveness() {
       }
     }
 
-    BBSet = NextBBSet;
+    BBSet = std::move(NextBBSet);
   }// while changed.
 }