i965/fs: Match destination type to size for ballot

No use in taking a 64-bit value when we know the high 32-bits are zero.

src/intel/compiler/brw_compiler.c

@@ -58,7 +58,7 @@ static const struct nir_shader_compiler_options scalar_nir_options = {
    .lower_unpack_unorm_2x16 = true,
    .lower_unpack_unorm_4x8 = true,
    .lower_subgroup_masks = true,
-   .max_subgroup_size = 64, /* FIXME */
+   .max_subgroup_size = 32,
    .max_unroll_iterations = 32,
 };