Index: llvm/trunk/include/llvm/Analysis/ConstantFolding.h
===================================================================
--- llvm/trunk/include/llvm/Analysis/ConstantFolding.h
+++ llvm/trunk/include/llvm/Analysis/ConstantFolding.h
@@ -102,6 +102,13 @@
 Constant *ConstantFoldExtractValueInstruction(Constant *Agg,
                                               ArrayRef<unsigned> Idxs);
 
+/// \brief Attempt to constant fold an insertelement instruction with the
+/// specified operands and indices.  The constant result is returned if
+/// successful; if not, null is returned.
+Constant *ConstantFoldInsertElementInstruction(Constant *Val,
+                                               Constant *Elt,
+                                               Constant *Idx);
+
 /// \brief Attempt to constant fold an extractelement instruction with the
 /// specified operands and indices.  The constant result is returned if
 /// successful; if not, null is returned.
Index: llvm/trunk/include/llvm/Analysis/InstructionSimplify.h
===================================================================
--- llvm/trunk/include/llvm/Analysis/InstructionSimplify.h
+++ llvm/trunk/include/llvm/Analysis/InstructionSimplify.h
@@ -161,6 +161,10 @@
 Value *SimplifyInsertValueInst(Value *Agg, Value *Val, ArrayRef<unsigned> Idxs,
                                const SimplifyQuery &Q);
 
+/// Given operands for an InsertElement, fold the result or return null.
+Value *SimplifyInsertElementInst(Value *Vec, Value *Elt, Value *Idx,
+                                 const SimplifyQuery &Q);
+
 /// Given operands for an ExtractValueInst, fold the result or return null.
 Value *SimplifyExtractValueInst(Value *Agg, ArrayRef<unsigned> Idxs,
                                 const SimplifyQuery &Q);
Index: llvm/trunk/lib/Analysis/InstructionSimplify.cpp
===================================================================
--- llvm/trunk/lib/Analysis/InstructionSimplify.cpp
+++ llvm/trunk/lib/Analysis/InstructionSimplify.cpp
@@ -3827,6 +3827,28 @@
   return ::SimplifyInsertValueInst(Agg, Val, Idxs, Q, RecursionLimit);
 }
 
+Value *llvm::SimplifyInsertElementInst(Value *Vec, Value *Val, Value *Idx,
+                                       const SimplifyQuery &Q) {
+  // Try to constant fold.
+  auto *VecC = dyn_cast<Constant>(Vec);
+  auto *ValC = dyn_cast<Constant>(Val);
+  auto *IdxC = dyn_cast<Constant>(Idx);
+  if (VecC && ValC && IdxC)
+    return ConstantFoldInsertElementInstruction(VecC, ValC, IdxC);
+
+  // Fold into undef if index is out of bounds.
+  if (auto *CI = dyn_cast<ConstantInt>(Idx)) {
+    uint64_t NumElements = cast<VectorType>(Vec->getType())->getNumElements();
+
+    if (CI->uge(NumElements))
+      return UndefValue::get(Vec->getType());
+  }
+
+  // TODO: We should also fold if index is iteslf an undef.
+
+  return nullptr;
+}
+
 /// Given operands for an ExtractValueInst, see if we can fold the result.
 /// If not, this returns null.
 static Value *SimplifyExtractValueInst(Value *Agg, ArrayRef<unsigned> Idxs,
@@ -4695,6 +4717,12 @@
                                      IV->getIndices(), Q);
     break;
   }
+  case Instruction::InsertElement: {
+    auto *IE = cast<InsertElementInst>(I);
+    Result = SimplifyInsertElementInst(IE->getOperand(0), IE->getOperand(1),
+                                       IE->getOperand(2), Q);
+    break;
+  }
   case Instruction::ExtractValue: {
     auto *EVI = cast<ExtractValueInst>(I);
     Result = SimplifyExtractValueInst(EVI->getAggregateOperand(),
Index: llvm/trunk/test/Transforms/InstSimplify/pr28725.ll
===================================================================
--- llvm/trunk/test/Transforms/InstSimplify/pr28725.ll
+++ llvm/trunk/test/Transforms/InstSimplify/pr28725.ll
@@ -1,6 +1,4 @@
 ; RUN: opt -S -instsimplify < %s | FileCheck %s
-target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
-target triple = "x86_64-pc-windows-msvc"
 %S = type { i16, i32 }
 
 define <2 x i16> @test1() {
@@ -9,5 +7,6 @@
   ret <2 x i16> %b
 }
 
+; InstCombine will be able to fold this into zeroinitializer
 ; CHECK-LABEL: @test1(
-; CHECK: ret <2 x i16> zeroinitializer
+; CHECK: ret <2 x i16> <i16 extractvalue (%S select (i1 icmp eq (i16 extractelement (<2 x i16> bitcast (<1 x i32> <i32 1> to <2 x i16>), i32 0), i16 0), %S zeroinitializer, %S { i16 0, i32 1 }), 0), i16 0>