[SPIR-V] DXC crashes during the internal `spirv-opt` validation pass.

Description
When compiling an HLSL compute shader to SPIR-V targeting Vulkan 1.1 environment, DXC crashes during the internal spirv-opt validation pass.

The crash occurs when custom explicit SPIR-V opcodes mapping via [vk::ext_instruction(333)] is combined with standard wave control-flow builtins like WaveIsFirstLane() and WaveReadLaneFirst() interacting with an indexed structured buffer array parameter. The internal optimizer emits a malformed instruction length header for sub-group instructions, leading to validation termination.

Compiler Flags used:

-T cs_6_6 -spirv -fspv-target-env=vulkan1.1 -E main

Steps to Reproduce
source hlsl:

struct DrawIndexedIndirectCommand
{
	uint indexCount;
	uint instanceCount;
	uint firstIndex;
	int vertexOffset;
	uint firstInstance;
};

struct CBData
{
	uint numInstanceObjects;
};

cbuffer CBuffer
{
	CBData cbData;
};
[[vk::binding(1, 0)]] RWStructuredBuffer<DrawIndexedIndirectCommand> numObjects : register(u0);
[[vk::binding(2, 0)]] RWStructuredBuffer<uint> instanceId : register(u1);

[[vk::ext_instruction(333)]]
uint subgroup_add(uint scope, uint operation, uint value);
#define WavePrefixCountBits(visible_bool) subgroup_add(3, 1, uint(visible_bool))
#define WaveActiveCountBits(visible_bool) subgroup_add(3, 0, uint(visible_bool))

inline float SignedDistanceToPoint(in float4 plane, in float3 p)
{
	return dot(plane.xyz, p) + plane.w;
}

[numthreads(32, 1, 1)]
void main(uint3 DTid : SV_DispatchThreadID, uint group_index : SV_GroupIndex)
{
	uint index = DTid.x;
	if (index >= cbData.numInstanceObjects)
	{
		return;
	}
	bool visible = (index % 2u) == 0u;
	uint local_index = WavePrefixCountBits(visible);
	uint wave_count = WaveActiveCountBits(visible);
	uint wave_offset = 0;
	if (wave_count && WaveIsFirstLane())
	{
		InterlockedAdd(numObjects[0].instanceCount, wave_count, wave_offset);
	}
	wave_offset = WaveReadLaneFirst(wave_offset);
	if (visible)
	{
		instanceId[wave_offset + local_index] = index;
	}
}

Actual Behavior
fatal error: failed to optimize SPIR-V: Invalid instruction OpGroupNonUniformElect starting at word 1707: expected no more operands after 4 words, but stated word count is 6.
note: please file a bug report on https://github.com/Microsoft/DirectXShaderCompiler/issues with source code if possible

Compiler returned: 5

Environment

• DXC version trunk, 1.10.2605.24, last version, 1.9(5402-0d3ee6b5)(1.9.0.5402) - 1.9.0.5402 (0d3ee6b55-dirty)
• Windows 11

Start by running the supplied HLSL reproducer with DXC using the listed Vulkan 1.1 and SPIR-V flags, then inspect the SPIR-V optimization and validation path for the malformed OpGroupNonUniformElect instruction. Done means the reproducer compiles without the internal spirv-opt failure and produces valid SPIR-V.