spirv2dxil: When removing unused inputs, make sure they're actually inputs

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20614>

src/microsoft/spirv_to_dxil/dxil_spirv_nir.c

@@ -490,7 +490,7 @@ kill_undefined_varyings(struct nir_builder *b,
       return false;
 
    nir_variable *var = nir_intrinsic_get_var(intr, 0);
-   if (!var)
+   if (!var || var->data.mode != nir_var_shader_in)
       return false;
 
    /* Ignore builtins for now, some of them get default values