[DomTreeUpdater] Add all insert before all delete updates to reduce compile time.

Summary:
The cleanup in D62751 introduced a compile-time regression due to the way DT updates are performed.
Add all insert edges then all delete edges in DTU to match the previous compile time.
Compile time on the test provided by @mstorsjo before and after this patch on my machine:
113.046s vs 35.649s
Repro: clang -target x86_64-w64-mingw32 -c -O3 glew-preproc.c; on https://martin.st/temp/glew-preproc.c.

Reviewers: kuhar, NutshellySima, mstorsjo

Subscribers: jlebar, mstorsjo, dmgreen, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62981

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@362839 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Utils/BasicBlockUtils.cpp

@@ -212,13 +212,19 @@ bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, DomTreeUpdater *DTU,
   std::vector<DominatorTree::UpdateType> Updates;
   if (DTU) {
     Updates.reserve(1 + (2 * succ_size(BB)));
-    Updates.push_back({DominatorTree::Delete, PredBB, BB});
-    for (auto I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
-      Updates.push_back({DominatorTree::Delete, BB, *I});
+    // Add insert edges first. Experimentally, for the particular case of two
+    // blocks that can be merged, with a single successor and single predecessor
+    // respectively, it is beneficial to have all insert updates first. Deleting
+    // edges first may lead to unreachable blocks, followed by inserting edges
+    // making the blocks reachable again. Such DT updates lead to high compile
+    // times. We add inserts before deletes here to reduce compile time.
+    for (auto I = succ_begin(BB), E = succ_end(BB); I != E; ++I)
       // This successor of BB may already have PredBB as a predecessor.
       if (llvm::find(successors(PredBB), *I) == succ_end(PredBB))
         Updates.push_back({DominatorTree::Insert, PredBB, *I});
-    }
+    for (auto I = succ_begin(BB), E = succ_end(BB); I != E; ++I)
+      Updates.push_back({DominatorTree::Delete, BB, *I});
+    Updates.push_back({DominatorTree::Delete, PredBB, BB});
   }
 
   if (MSSAU)