[MemorySSA] When applying updates, clean unnecessary Phis.

[asbirlea]

Summary

After applying a set of insert updates, there may be trivial Phis left over. Clean them up.

[george.burgess.iv]

LGTM -- thanks!

Please also add a test before committing if doing so is reasonably possible. If the test is super convoluted, I'm OK with this as-is.

[asbirlea]

Add test.

lib/Analysis/MemorySSAUpdater.cpp

for (auto &BBPredPair : PredMap) {
     }
   }
   // Nothing to process for new/cloned blocks.
   for (auto *BB : NewBlocks)
     PredMap.erase(BB);
 
   SmallVector<BasicBlock *, 8> BlocksToProcess;
   SmallVector<BasicBlock *, 16> BlocksWithDefsToReplace;
+  SmallVector<WeakVH, 8> InsertedPhis;
 
   // First create MemoryPhis in all blocks that don't have one. Create in the
   // order found in Updates, not in PredMap, to get deterministic numbering.
   for (auto &Edge : Updates) {
     BasicBlock *BB = Edge.getTo();
     if (PredMap.count(BB) && !MSSA->getMemoryAccess(BB))
-      MSSA->createMemoryPhi(BB);
+      InsertedPhis.push_back(MSSA->createMemoryPhi(BB));
   }
 
   // Now we'll fill in the MemoryPhis with the right incoming values.
   for (auto &BBPredPair : PredMap) {
     auto *BB = BBPredPair.first;
     const auto &PrevBlockSet = BBPredPair.second.Prev;
     const auto &AddedBlockSet = BBPredPair.second.Added;
     assert(!PrevBlockSet.empty() &&
if (!BlocksToProcess.empty()) {
     for (auto *BBIDF : IDFBlocks) {
       if (auto *IDFPhi = MSSA->getMemoryAccess(BBIDF)) {
         // Update existing Phi.
         // FIXME: some updates may be redundant, try to optimize and skip some.
         for (unsigned I = 0, E = IDFPhi->getNumIncomingValues(); I < E; ++I)
           IDFPhi->setIncomingValue(I, GetLastDef(IDFPhi->getIncomingBlock(I)));
       } else {
         IDFPhi = MSSA->createMemoryPhi(BBIDF);
+        InsertedPhis.push_back(IDFPhi);
         for (auto &Pair : children<GraphDiffInvBBPair>({GD, BBIDF})) {
           BasicBlock *Pi = Pair.second;
           IDFPhi->addIncoming(GetLastDef(Pi), Pi);
         }
       }
     }
   }
 
if (auto DefsList = MSSA->getWritableBlockDefs(BlockWithDefsToReplace)) {
               }
               cast<MemoryUseOrDef>(Usr)->resetOptimized();
             }
           }
         }
       }
     }
   }
+  tryRemoveTrivialPhis(InsertedPhis);
 }
 
 // Move What before Where in the MemorySSA IR.
 template <class WhereType>
 void MemorySSAUpdater::moveTo(MemoryUseOrDef *What, BasicBlock *BB,
                               WhereType Where) {
   // Mark MemoryPhi users of What not to be optimized.
   for (auto *U : What->users())