[DominanceFrontier] fix addToFrontier to use insert

[wenju]

Summary

No new lits are added since addToFrontier is only used in MachineDominanceFrontier::addToFrontier.

[arsenm]

MachineDominanceFrontier::addToFrontier.

Where is this used?

[wenju]

In D155417#4552384, @arsenm wrote:

MachineDominanceFrontier::addToFrontier.

Where is this used?

MachineDominanceFrontier::addToFrontier isn't used. I don't have downstream user of it either.
I notice this is probably a typo in DominanceFrontierBase::addToFrontier while investigating another issue.

[arsenm]

I guess if it's only downstream users we just have to trust it. It is suspicious that an "addTo" function was doing an erase

llvm/include/llvm/Analysis/DominanceFrontierImpl.h

for (iterator I = begin(), E = end(); I != E; ++I)
     I->second.erase(BB);
   Frontiers.erase(BB);
 }
 
 template <class BlockT, bool IsPostDom>
 void DominanceFrontierBase<BlockT, IsPostDom>::addToFrontier(iterator I,
                                                              BlockT *Node) {
   assert(I != end() && "BB is not in DominanceFrontier!");
-  assert(I->second.count(Node) && "Node is not in DominanceFrontier of BB");
-  I->second.erase(Node);
+  I->second.insert(Node);
 }
 
 template <class BlockT, bool IsPostDom>
 void DominanceFrontierBase<BlockT, IsPostDom>::removeFromFrontier(
     iterator I, BlockT *Node) {
   assert(I != end() && "BB is not in DominanceFrontier!");
   assert(I->second.count(Node) && "Node is not in DominanceFrontier of BB");
   I->second.erase(Node);