Update SetVector to rely on the underlying set's insert to return a pair<iterator, bool>

This is to be consistent with StringSet and ultimately with the standard
library's associative container insert function.

This lead to updating SmallSet::insert to return pair<iterator, bool>,
and then to update SmallPtrSet::insert to return pair<iterator, bool>,
and then to update all the existing users of those functions...

llvm-svn: 222334

llvm/lib/CodeGen/MachineTraceMetrics.cpp

@@ -449,7 +449,7 @@ public:
     }
     // To is a new block. Mark the block as visited in case the CFG has cycles
     // that MachineLoopInfo didn't recognize as a natural loop.
-    return LB.Visited.insert(To);
+    return LB.Visited.insert(To).second;
   }
 };
 }