Index: lib/Analysis/MemorySSAUpdater.cpp
===================================================================
--- lib/Analysis/MemorySSAUpdater.cpp
+++ lib/Analysis/MemorySSAUpdater.cpp
@@ -858,7 +858,6 @@
   for (auto *BB : NewBlocks)
     PredMap.erase(BB);
 
-  SmallVector<BasicBlock *, 8> BlocksToProcess;
   SmallVector<BasicBlock *, 16> BlocksWithDefsToReplace;
   SmallVector<WeakVH, 8> InsertedPhis;
 
@@ -934,10 +933,6 @@
       for (auto *Pred : PrevBlockSet)
         for (int I = 0, E = EdgeCountMap[{Pred, BB}]; I < E; ++I)
           NewPhi->addIncoming(DefP1, Pred);
-
-      // Insert BB in the set of blocks that now have definition. We'll use this
-      // to compute IDF and add Phis there next.
-      BlocksToProcess.push_back(BB);
     }
 
     // Get all blocks that used to dominate BB and no longer do after adding
@@ -952,6 +947,14 @@
     GetNoLongerDomBlocks(PrevIDom, NewIDom, BlocksWithDefsToReplace);
   }
 
+  tryRemoveTrivialPhis(InsertedPhis);
+  // Create the set of blocks that now have a definition. We'll use this to
+  // compute IDF and add Phis there next.
+  SmallVector<BasicBlock *, 8> BlocksToProcess;
+  for (auto &VH : InsertedPhis)
+    if (auto *MPhi = cast_or_null<MemoryPhi>(VH))
+      BlocksToProcess.push_back(MPhi->getBlock());
+
   // Compute IDF and add Phis in all IDF blocks that do not have one.
   SmallVector<BasicBlock *, 32> IDFBlocks;
   if (!BlocksToProcess.empty()) {
Index: test/Analysis/MemorySSA/simple_loop_unswitch_nontrivial.ll
===================================================================
--- /dev/null
+++ test/Analysis/MemorySSA/simple_loop_unswitch_nontrivial.ll
@@ -0,0 +1,113 @@
+; RUN: opt -simple-loop-unswitch -enable-nontrivial-unswitch -verify-memoryssa -S %s | FileCheck %s
+; REQUIRES: asserts
+
+target triple = "x86_64-unknown-linux-gnu"
+
+declare void @foo()
+
+; In Test1, there are no definitions. MemorySSA updates insert trivial phis and remove them.
+; Verify all are removed, considering the SLU pass leaves unreachable blocks hanging when the MSSA updates are done.
+; CHECK-LABEL: @Test1
+define void @Test1(i32) {
+header:
+  br label %outer
+
+outer.loopexit.split:                             ; preds = %continue
+  br label %outer.loopexit
+
+outer.loopexit:                                   ; preds = %outer.loopexit.split.us, %outer.loopexit.split
+  br label %outer
+
+outer:                                            ; preds = %outer.loopexit, %header
+  br i1 false, label %outer.split.us, label %outer.split
+
+outer.split.us:                                   ; preds = %outer
+  br label %inner.us
+
+inner.us:                                         ; preds = %continue.us, %outer.split.us
+  br label %overflow.us
+
+overflow.us:                                      ; preds = %inner.us
+  br label %continue.us
+
+continue.us:                                      ; preds = %overflow.us
+  br i1 true, label %outer.loopexit.split.us, label %inner.us
+
+outer.loopexit.split.us:                          ; preds = %continue.us
+  br label %outer.loopexit
+
+outer.split:                                      ; preds = %outer
+  br label %inner
+
+inner:                                            ; preds = %continue, %outer.split
+  br label %switchme
+
+switchme:                                         ; preds = %inner
+  switch i32 %0, label %continue [
+    i32 88, label %go_out
+    i32 99, label %case2
+  ]
+
+case2:                                            ; preds = %switchme
+  br label %continue
+
+continue:                                         ; preds = %case2, %switchme
+  br i1 true, label %outer.loopexit.split, label %inner
+
+go_out:                                           ; preds = %switchme
+  unreachable
+}
+
+; In Test2 there is a single def (call to foo). There are already Phis in place that are cloned when unswitching.
+; Ensure MemorySSA remains correct. Due to SLU's pruned cloning, continue.us2 becomes unreachable, with an empty Phi that is later cleaned.
+; CHECK-LABEL: @Test2
+define void @Test2(i32) {
+header:
+  br label %outer
+
+outer.loopexit.split:                             ; preds = %continue
+  br label %outer.loopexit
+
+outer.loopexit:                                   ; preds = %outer.loopexit.split.us, %outer.loopexit.split
+  br label %outer
+
+outer:                                            ; preds = %outer.loopexit, %header
+  br i1 false, label %outer.split.us, label %outer.split
+
+outer.split.us:                                   ; preds = %outer
+  br label %inner.us
+
+inner.us:                                         ; preds = %continue.us, %outer.split.us
+  br label %overflow.us
+
+overflow.us:                                      ; preds = %inner.us
+  br label %continue.us
+
+continue.us:                                      ; preds = %overflow.us
+  br i1 true, label %outer.loopexit.split.us, label %inner.us
+
+outer.loopexit.split.us:                          ; preds = %continue.us
+  br label %outer.loopexit
+
+outer.split:                                      ; preds = %outer
+  br label %inner
+
+inner:                                            ; preds = %continue, %outer.split
+  br label %switchme
+
+switchme:                                         ; preds = %inner
+  switch i32 %0, label %continue [
+    i32 88, label %go_out
+    i32 99, label %case2
+  ]
+
+case2:                                            ; preds = %switchme
+  call void @foo()
+  br label %continue
+
+continue:                                         ; preds = %case2, %switchme
+  br i1 true, label %outer.loopexit.split, label %inner
+
+go_out:                                           ; preds = %switchme
+  unreachable
+}