DCE does not remove unused built-in Input variables, keeping required capabilities alive

[Przemog1]

SPIRV-Tools DCE fails to remove unused built-in Input variables.
As a result, capabilities required by those built-ins are kept, even though the variables
are never referenced anywhere in the module.

Steps to reproduce:
HLSL: https://godbolt.org/z/a6TsvPE9c
SPIR-V emitted: https://godbolt.org/z/Ks7915d63

Additional note:
This is likely caused by the fact that SPV_EXT_shader_atomic_float16_add is not included in the DCE whitelist.