[MemorySSA] Use GraphDiff info when computing IDF.

[asbirlea]

Summary

When computing IDF for insert updates, ensure we use the snapshot CFG offered by GraphDiff.
Caught by D63389.

[george.burgess.iv]

Thanks!

llvm/trunk/lib/Analysis/MemorySSAUpdater.cpp

for (auto &BBPredPair : PredMap) {
     assert(DT.dominates(NewIDom, PrevIDom) &&
            "New idom should dominate old idom");
     GetNoLongerDomBlocks(PrevIDom, NewIDom, BlocksWithDefsToReplace);
   }
 
   // Compute IDF and add Phis in all IDF blocks that do not have one.
   SmallVector<BasicBlock *, 32> IDFBlocks;
   if (!BlocksToProcess.empty()) {
-    ForwardIDFCalculator IDFs(DT);
+    ForwardIDFCalculator IDFs(DT, GD);
     SmallPtrSet<BasicBlock *, 16> DefiningBlocks(BlocksToProcess.begin(),
                                                  BlocksToProcess.end());
     IDFs.setDefiningBlocks(DefiningBlocks);
     IDFs.calculate(IDFBlocks);
 
     SmallSetVector<MemoryPhi *, 4> PhisToFill;
     // First create all needed Phis.
     for (auto *BBIDF : IDFBlocks)