Index: lib/Transforms/Vectorize/SLPVectorizer.cpp
===================================================================
--- lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -246,6 +246,8 @@
 /// %ins4 = insertelement <4 x i8> %ins3, i8 %9, i32 3
 /// ret <4 x i8> %ins4
 /// InstCombiner transforms this into a shuffle and vector mul
+/// TODO: Can we split off and reuse the shuffle mask detection from
+/// TargetTransformInfo::getInstructionThroughput?
 static Optional<TargetTransformInfo::ShuffleKind>
 isShuffle(ArrayRef<Value *> VL) {
   auto *EI0 = cast<ExtractElementInst>(VL[0]);
@@ -272,7 +274,11 @@
       continue;
     // For correct shuffling we have to have at most 2 different vector operands
     // in all extractelement instructions.
-    if (Vec1 && Vec2 && Vec != Vec1 && Vec != Vec2)
+    if (!Vec1 || Vec1 == Vec)
+      Vec1 = Vec;
+    else if (!Vec2 || Vec2 == Vec)
+      Vec2 = Vec;
+    else
       return None;
     if (CommonShuffleMode == Permute)
       continue;
@@ -283,10 +289,6 @@
       continue;
     }
     // Check the shuffle mode for the current operation.
-    if (!Vec1)
-      Vec1 = Vec;
-    else if (Vec != Vec1)
-      Vec2 = Vec;
     // Example: shufflevector A, B, <0,5,2,7>
     // I is odd and IntIdx for A == I - FirstAlternate shuffle.
     // I is even and IntIdx for B == I - FirstAlternate shuffle.