nir/opt_uniform_subgroup: preserve divergence during optimization

nir_def_init sets divergent = true, this means for something like
reduce(reduce(convergent)) we previously only optimized the inner
reduce.

No fossil changes at the moment, but I hit this when trying to
optimize shared memory to subgroup operations.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/41542>

src/compiler/nir/nir_opt_uniform_subgroup.c

@@ -323,6 +323,12 @@ opt_uniform_subgroup_instr(nir_builder *b, nir_intrinsic_instr *intrin, void *_s
       return false;
    }
 
+
+   /* Divergence shouldn't change from the optimization that we have done,
+    * so preserve it in case another subgroup operation uses the result.
+    */
+   replacement->divergent = intrin->def.divergent;
+
    nir_def_replace(&intrin->def, replacement);
    return true;
 }