When per-sample shading is forced and all input variables have a flat
interpolation, DXIL validation detects a mismatch between the
SampleFrequency property and the fact that no variables are per-sample
and SV_SampleIndex is never read. When that happens, add a dummy
SV_SampleIndex.
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15916>