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...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222334 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/MachineDominators.h

@@ -303,7 +303,7 @@ public:
   void recordSplitCriticalEdge(MachineBasicBlock *FromBB,
                               MachineBasicBlock *ToBB,
                               MachineBasicBlock *NewBB) {
-    bool Inserted = NewBBs.insert(NewBB);
+    bool Inserted = NewBBs.insert(NewBB).second;
     (void)Inserted;
     assert(Inserted &&
            "A basic block inserted via edge splitting cannot appear twice");