[DirectX] Change placeholders from undef to poison

Call `insertelement` with a `poison` vector.

Author: pedroclobo

llvm/lib/Target/DirectX/DXILIntrinsicExpansion.cpp

@@ -147,7 +147,7 @@ static Value *expandCrossIntrinsic(CallInst *Orig) {
   Value *zx_xz = MulSub(op0_z, op0_x, op1_z, op1_x);
   Value *xy_yx = MulSub(op0_x, op0_y, op1_x, op1_y);
 
-  Value *cross = UndefValue::get(VT);
+  Value *cross = PoisonValue::get(VT);
   cross = Builder.CreateInsertElement(cross, yz_zy, (uint64_t)0);
   cross = Builder.CreateInsertElement(cross, zx_xz, 1);
   cross = Builder.CreateInsertElement(cross, xy_yx, 2);

llvm/lib/Target/DirectX/DXILOpLowering.cpp

@@ -483,7 +483,7 @@ class OpLowerer {
         if (!Extracts[I])
           Extracts[I] = IRB.CreateExtractValue(Op, I);
 
-      Value *Vec = UndefValue::get(OldTy);
+      Value *Vec = PoisonValue::get(OldTy);
       for (int I = 0, E = N; I != E; ++I)
         Vec = IRB.CreateInsertElement(Vec, Extracts[I], I);
       OldResult->replaceAllUsesWith(Vec);