microsoft/compiler: Emit a flat interpolation method for SV_SampleIndex

We do not want to set an interpolation method for vertex shader inputs.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12513>

src/microsoft/compiler/dxil_signature.c

@@ -191,6 +191,7 @@ get_semantic_sv_name(nir_variable *var, struct semantic_info *info, bool _vulkan
       break;
    case SYSTEM_VALUE_SAMPLE_ID:
       info->kind = DXIL_SEM_SAMPLE_INDEX;
+      info->interpolation = get_interpolation(var);
       break;
    default:
       unreachable("unsupported system value");