Index: lib/Analysis/InstructionSimplify.cpp
===================================================================
--- lib/Analysis/InstructionSimplify.cpp
+++ lib/Analysis/InstructionSimplify.cpp
@@ -4118,6 +4118,11 @@
   if (Op0Const && Op1Const)
     return ConstantFoldShuffleVectorInstruction(Op0Const, Op1Const, Mask);
 
+  // A shuffle of a splat is always the splat itself
+  if (auto Op0Shuf = dyn_cast<ShuffleVectorInst>(Op0))
+    if (isa<UndefValue>(Op1) && Op0Shuf->getMask()->getSplatValue())
+      return Op0;
+
   return nullptr;
 }
 
Index: test/Transforms/InstSimplify/shufflevector.ll
===================================================================
--- test/Transforms/InstSimplify/shufflevector.ll
+++ test/Transforms/InstSimplify/shufflevector.ll
@@ -4,14 +4,23 @@
 define <4 x i32> @splat_operand(<4 x i32> %x) {
 ; CHECK-LABEL: @splat_operand(
 ; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <4 x i32> [[X:%.*]], <4 x i32> undef, <4 x i32> zeroinitializer
-; CHECK-NEXT:    [[SHUF:%.*]] = shufflevector <4 x i32> [[SPLAT]], <4 x i32> undef, <4 x i32> <i32 0, i32 3, i32 2, i32 1>
-; CHECK-NEXT:    ret <4 x i32> [[SHUF]]
+; CHECK-NEXT:    ret <4 x i32> [[SPLAT]]
 ;
   %splat = shufflevector <4 x i32> %x, <4 x i32> undef, <4 x i32> zeroinitializer
   %shuf = shufflevector <4 x i32> %splat, <4 x i32> undef, <4 x i32> <i32 0, i32 3, i32 2, i32 1>
   ret <4 x i32> %shuf
 }
 
+define <4 x i32> @splat_operand1(<4 x i32> %x, <4 x i32> %y) {
+; CHECK-LABEL: @splat_operand1(
+; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <4 x i32> [[X:%.*]], <4 x i32> [[Y:%.*]], <4 x i32> zeroinitializer
+; CHECK-NEXT:    ret <4 x i32> [[SPLAT]]
+;
+  %splat = shufflevector <4 x i32> %x, <4 x i32> %y, <4 x i32> zeroinitializer
+  %shuf = shufflevector <4 x i32> %splat, <4 x i32> undef, <4 x i32> <i32 0, i32 3, i32 2, i32 1>
+  ret <4 x i32> %shuf
+}
+
 define <4 x i32> @undef_mask(<4 x i32> %x) {
 ; CHECK-LABEL: @undef_mask(
 ; CHECK-NEXT:    ret <4 x i32> undef