[MemorySSAUpdater] Simplify updates when only deleting edges.

When performing only edge deletion, we don't need to do the DT updates
back and forth. Check for the existance of insert updates to simplify
this.

Author: alinas

llvm/lib/Analysis/MemorySSAUpdater.cpp

@@ -822,25 +822,30 @@ void MemorySSAUpdater::applyUpdates(ArrayRef<CFGUpdate> Updates,
   }
 
   if (!DeleteUpdates.empty()) {
-    if (!UpdateDT) {
-      SmallVector<CFGUpdate, 0> Empty;
-      // Deletes are reversed applied, because this CFGView is pretending the
-      // deletes did not happen yet, hence the edges still exist.
-      DT.applyUpdates(Empty, RevDeleteUpdates);
+    if (!InsertUpdates.empty()) {
+      if (!UpdateDT) {
+        SmallVector<CFGUpdate, 0> Empty;
+        // Deletes are reversed applied, because this CFGView is pretending the
+        // deletes did not happen yet, hence the edges still exist.
+        DT.applyUpdates(Empty, RevDeleteUpdates);
+      } else {
+        // Apply all updates, with the RevDeleteUpdates as PostCFGView.
+        DT.applyUpdates(Updates, RevDeleteUpdates);
+      }
+
+      // Note: the MSSA update below doesn't distinguish between a GD with
+      // (RevDelete,false) and (Delete, true), but this matters for the DT
+      // updates above; for "children" purposes they are equivalent; but the
+      // updates themselves convey the desired update, used inside DT only.
+      GraphDiff<BasicBlock *> GD(RevDeleteUpdates);
+      applyInsertUpdates(InsertUpdates, DT, &GD);
+      // Update DT to redelete edges; this matches the real CFG so we can
+      // perform the standard update without a postview of the CFG.
+      DT.applyUpdates(DeleteUpdates);
     } else {
-      // Apply all updates, with the RevDeleteUpdates as PostCFGView.
-      DT.applyUpdates(Updates, RevDeleteUpdates);
+      if (UpdateDT)
+        DT.applyUpdates(DeleteUpdates);
     }
-
-    // Note: the MSSA update below doesn't distinguish between a GD with
-    // (RevDelete,false) and (Delete, true), but this matters for the DT
-    // updates above; for "children" purposes they are equivalent; but the
-    // updates themselves convey the desired update, used inside DT only.
-    GraphDiff<BasicBlock *> GD(RevDeleteUpdates);
-    applyInsertUpdates(InsertUpdates, DT, &GD);
-    // Update DT to redelete edges; this matches the real CFG so we can perform
-    // the standard update without a postview of the CFG.
-    DT.applyUpdates(DeleteUpdates);
   } else {
     if (UpdateDT)
       DT.applyUpdates(Updates);