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

lib/Analysis/PtrUseVisitor.cpp

@@ -17,7 +17,7 @@ using namespace llvm;
 
 void detail::PtrUseVisitorBase::enqueueUsers(Instruction &I) {
   for (Use &U : I.uses()) {
-    if (VisitedUses.insert(&U)) {
+    if (VisitedUses.insert(&U).second) {
       UseToVisit NewU = {
         UseToVisit::UseAndIsOffsetKnownPair(&U, IsOffsetKnown),
         Offset