Index: llvm/lib/Target/AMDGPU/AMDGPUInstCombineIntrinsic.cpp
===================================================================
--- llvm/lib/Target/AMDGPU/AMDGPUInstCombineIntrinsic.cpp
+++ llvm/lib/Target/AMDGPU/AMDGPUInstCombineIntrinsic.cpp
@@ -385,6 +385,9 @@
   APInt DemandedElts = APInt::getAllOnes(VWidth);
 
   for (int i = VWidth - 1; i > 0; --i) {
+    // Don't call findScalarElement unless UseV has a definition.
+    auto *Elt = isa<Instruction>(UseV) ? findScalarElement(UseV, i) : UseV;
+
     APInt DemandOneElt = APInt::getOneBitSet(VWidth, i);
     KnownFPClass KnownFPClass =
         computeKnownFPClass(UseV, DemandOneElt, IC.getDataLayout(),
@@ -392,8 +395,9 @@
                             /*Depth=*/0, &IC.getTargetLibraryInfo(),
                             &IC.getAssumptionCache(), I,
                             &IC.getDominatorTree());
-    if (KnownFPClass.KnownFPClasses != fcPosZero)
+    if (KnownFPClass.KnownFPClasses != fcPosZero && !isa<UndefValue>(Elt))
       break;
+
     DemandedElts.clearBit(i);
   }
   return DemandedElts;
Index: llvm/test/Transforms/InstCombine/AMDGPU/amdgcn-simplify-image-buffer-stores.ll
===================================================================
--- llvm/test/Transforms/InstCombine/AMDGPU/amdgcn-simplify-image-buffer-stores.ll
+++ llvm/test/Transforms/InstCombine/AMDGPU/amdgcn-simplify-image-buffer-stores.ll
@@ -84,6 +84,19 @@
   ret void
 }
 
+define amdgpu_ps void @struct_tbuffer_store_insert_undefs(<4 x i32> inreg %a, float %vdata1, i32 %b) {
+; GCN-LABEL: @struct_tbuffer_store_insert_undefs(
+; GCN-NEXT:    [[TMP1:%.*]] = insertelement <2 x float> <float poison, float 1.000000e+00>, float [[VDATA1:%.*]], i64 0
+; GCN-NEXT:    call void @llvm.amdgcn.struct.tbuffer.store.v2f32(<2 x float> [[TMP1]], <4 x i32> [[A:%.*]], i32 [[B:%.*]], i32 0, i32 42, i32 0, i32 15)
+; GCN-NEXT:    ret void
+;
+  %newvdata1 = insertelement <4 x float> undef, float %vdata1, i32 0
+  %newvdata2 = insertelement <4 x float> %newvdata1, float 1.0, i32 1
+  call void @llvm.amdgcn.struct.tbuffer.store.v4f32(<4 x float> %newvdata2, <4 x i32> %a, i32 %b, i32 0, i32 42, i32 0, i32 15)
+  ret void
+}
+
+
 declare void @llvm.amdgcn.raw.buffer.store.format.v4f32(<4 x float>, <4 x i32>, i32, i32, i32) #2
 declare void @llvm.amdgcn.buffer.store.format.v4f32(<4 x float>, <4 x i32>, i32, i32, i1, i1) #1
 declare void @llvm.amdgcn.struct.buffer.store.format.v4f32(<4 x float>, <4 x i32>, i32, i32, i32, i32) #2