Index: llvm/lib/Analysis/LoopInfo.cpp
===================================================================
--- llvm/lib/Analysis/LoopInfo.cpp
+++ llvm/lib/Analysis/LoopInfo.cpp
@@ -211,24 +211,18 @@
     // Go through each predecessor of the loop header and check the
     // terminator for the metadata.
     BasicBlock *H = getHeader();
-    for (BasicBlock *BB : this->blocks()) {
-      TerminatorInst *TI = BB->getTerminator();
-      MDNode *MD = nullptr;
-
+    for (BasicBlock *BB : predecessors(H)) {
       // Check if this terminator branches to the loop header.
-      for (BasicBlock *Successor : TI->successors()) {
-        if (Successor == H) {
-          MD = TI->getMetadata(LLVMContext::MD_loop);
-          break;
-        }
-      }
-      if (!MD)
-        return nullptr;
+      if (!this->contains(BB))
+        continue;
 
-      if (!LoopID)
-        LoopID = MD;
-      else if (MD != LoopID)
-        return nullptr;
+      TerminatorInst *TI = BB->getTerminator();
+      if (MDNode *MD = TI->getMetadata(LLVMContext::MD_loop)) {
+        if (!LoopID)
+          LoopID = MD;
+        else if (MD != LoopID) // Multiple MD_loop found => corrupt metadata.
+          return nullptr;
+      }
     }
   }
   if (!LoopID || LoopID->getNumOperands() == 0 ||
@@ -248,12 +242,16 @@
   }
 
   BasicBlock *H = getHeader();
-  for (BasicBlock *BB : this->blocks()) {
+  bool MDSet = false;
+  for (BasicBlock *BB : predecessors(H)) {
+    // Check if this terminator branches to the loop header.
+    if (!this->contains(BB))
+      continue;
+
     TerminatorInst *TI = BB->getTerminator();
-    for (BasicBlock *Successor : TI->successors()) {
-      if (Successor == H)
-        TI->setMetadata(LLVMContext::MD_loop, LoopID);
-    }
+    TI->setMetadata(LLVMContext::MD_loop, LoopID);
+    assert(!MDSet && "Multiple branches to loop header from within loop.");
+    MDSet = true;
   }
 }