This is related to microsoft/DirectXShaderCompiler#2381.

When DXC compiles vtx_input.hlsl to vtx_input.spv.fcgl, it emits temporary variables to handle the entry function.
When we run spirv-opt to inline the entry funtion, those temporary variables still exist.
(Please check vtx_input.spv.opt for the output of spirv-opt)

vtx_input.hlsl.txt
vtx_input.spv.fcgl.txt
vtx_input.spv.opt.txt