Index: lib/Transforms/InstCombine/InstCombineVectorOps.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineVectorOps.cpp
+++ lib/Transforms/InstCombine/InstCombineVectorOps.cpp
@@ -1142,9 +1142,9 @@
 
   bool MadeChange = false;
 
-  // Undefined shuffle mask -> undefined value.
-  if (isa<UndefValue>(SVI.getOperand(2)))
-    return replaceInstUsesWith(SVI, UndefValue::get(SVI.getType()));
+  if (auto *V = SimplifyShuffleVectorInst(LHS, RHS, SVI.getMask(),
+                                          SVI.getType(), DL, &TLI, &DT, &AC))
+    return replaceInstUsesWith(SVI, V);
 
   unsigned VWidth = SVI.getType()->getVectorNumElements();