intel/fs: rerun divergence prior to lowering non-uniform interpolate at sample

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: 74a40cc4 ("intel/fs: move lower of non-uniform at_sample barycentric to NIR")
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <mesa/mesa!26797>