Index: llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
@@ -2004,7 +2004,8 @@
   for (unsigned i = 0; i != NumElts; ++i)
     NewMask[i] = Mask[i] < (signed)NumElts ? Mask[i] : Mask1[i];
 
-  assert(ShuffleVectorInst::isSelectMask(NewMask) && "Unexpected shuffle mask");
+  // A select mask with undef elements might look like an identity mask.
+  assert((ShuffleVectorInst::isSelectMask(NewMask) || ShuffleVectorInst::isIdentityMask(NewMask)) && "Unexpected shuffle mask");
   return new ShuffleVectorInst(X, Y, NewMask);
 }
 
Index: llvm/test/Transforms/InstCombine/foldSelectShuffleOfSelectShuffle.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/InstCombine/foldSelectShuffleOfSelectShuffle.ll
@@ -0,0 +1,13 @@
+; RUN: opt -passes=instcombine %s -S -o - | FileCheck %s
+
+; CHECK: shufflevector <4 x float> %1, <4 x float> poison, <4 x i32> <i32 0, i32 undef, i32 2, i32 3>
+
+@g = dso_local local_unnamed_addr global [4 x float] zeroinitializer, align 16
+@h = dso_local local_unnamed_addr global [4 x float] zeroinitializer, align 16
+define <4 x float> @square() local_unnamed_addr #0 {
+  %1 = load <4 x float>, ptr @g, align 16
+  %2 = load <4 x float>, ptr @h, align 16
+  %3 = shufflevector <4 x float> %1, <4 x float> %2, <4 x i32> <i32 0, i32 5, i32 undef, i32 undef>
+  %4 = shufflevector <4 x float> %3, <4 x float> %1, <4 x i32> <i32 0, i32 undef, i32 6, i32 7>
+  ret <4 x float> %4
+}