[SPIR-V][HLSL] Fix lowering of poison in shufflevector when swizzle are used

[Keenuts]

Given the following sample

// RUN: %clang --driver-mode=dxc %s -T ps_6_8 -E main -O3 -spirv

/* Copyright (c) 2025, Sascha Willems
 *
 * SPDX-License-Identifier: Apache-2.0
 *
 * Licensed under the Apache License, Version 2.0 the "License";
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

struct VSOutput
{
    float4 Position : SV_POSITION;
    [[vk::location(0)]] float3 Color : COLOR0;
};

float4 main(VSOutput input) : SV_TARGET
{
    return float4(input.Color, 1.0);
}

The generated IR is:

  %0 = load <3 x float>, ptr addrspace(7) @COLOR0, align 16
  %1 = shufflevector <3 x float> %0, <3 x float> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 poison>
  %vecinit78.i = insertelement <4 x float> %1, float 1.000000e+00, i64 3
  store <4 x float> %vecinit78.i, ptr addrspace(8) @SV_TARGET0, align 16

Issue is the poison being used in the shufflevector index.
This is lowered as a -1 in the backend, which causes a validation error.
Emitting any other valid index like 0 should be fine since the value is overriden just after.