diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -6527,19 +6527,21 @@
 
   if (C->isNullValue() || isa<UndefValue>(C)) {
     // Only look at the first use, avoid hurting compile time with long uselists
-    User *Use = *I->user_begin();
+    auto *Use = cast<Instruction>(*I->user_begin());
+    // Bail out if Use is not in the same BB as I or Use == I or Use comes
+    // before I in the block. The latter two can be the case if Use is a PHI
+    // node.
+    if (Use->getParent() != I->getParent() || Use == I || Use->comesBefore(I))
+      return false;
 
     // Now make sure that there are no instructions in between that can alter
     // control flow (eg. calls)
-    for (BasicBlock::iterator
-             i = ++BasicBlock::iterator(I),
-             UI = BasicBlock::iterator(dyn_cast<Instruction>(Use));
-         i != UI; ++i) {
-      if (i == I->getParent()->end())
-        return false;
-      if (!isGuaranteedToTransferExecutionToSuccessor(&*i))
-        return false;
-    }
+    auto InstrRange =
+        make_range(std::next(I->getIterator()), Use->getIterator());
+    if (any_of(InstrRange, [](Instruction &I) {
+          return !isGuaranteedToTransferExecutionToSuccessor(&I);
+        }))
+      return false;
 
     // Look through GEPs. A load from a GEP derived from NULL is still undefined
     if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Use))