[DXIL] Decorating CS float argument with SV_DispatchThreadID semantic crashes the compiler (float2, float3 and float4 works)

Title

[DXIL] Decorating CS float argument with SV_DispatchThreadID semantic crashes the compiler (float2, float3 and float4 works)

Functional impact

Compiler crashes

Minimal repro steps

1. Compile the following shader with dxc.exe -T cs_6_0 -E CSMain issue_shader.txt:

RWStructuredBuffer<float4> rwTexture;

[numthreads(1, 1, 1)]
void CSMain(float id : SV_DispatchThreadID)
{
	rwTexture[3] = id.xxxx;
}

Expected result

Shader compiles correctly, as it does for vector float arguments: float2, float3, float4

Actual result

Compiler crashes

Further technical details

Ran test on shader playground with DXC from 2020 12 16
The SPIR-V backend produces validation errors (reported here https://github.com/microsoft/DirectXShaderCompiler/issues/3443 )

Start by reproducing the crash with dxc.exe using the inline issue_shader.txt shader and compare the scalar float argument with the working float2, float3, and float4 cases. Trace the compiler path for the SV_DispatchThreadID semantic and verify that the shader compiles without crashing, while checking the noted SPIR-V backend validation behavior separately.