intel/fs: move lower of non-uniform at_sample barycentric to NIR

intel/fs: move lower of non-uniform at_sample barycentric to NIR

We use a non-uniform lowering loop in the backend which we can do
better in NIR because we can also use divergence analysis there.

This change also limits VGRF usage to a single VGRF to hold the sample
ID in the backend.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24716>