nir/lower_idiv: Remove imprecise_32bit_lowering

NIR has two implementations of lower_idiv, keyed on the
imprecise_32bit_lowering flag. This flag is misleading: the results when
setting this flag "imprecise", they're completely wrong for some values.
If a backend has a native implementation of umul_high, the correct path
isn't that much more expensive. If it doesn't, it's substantially slower
for highp integer divison... but in practice, non-constant highp integer
division is pretty rare.

After a painful migration of the tree, this code path has no more users.
Remove it so nobody else gets the bright idea of using it again.

Closes: #6555
Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19303>

src/intel/compiler/brw_nir.c

@@ -1254,7 +1254,6 @@ brw_postprocess_nir(nir_shader *nir, const struct brw_compiler *compiler,
       /* Lower integer division by constants before nir_lower_idiv. */
       OPT(nir_opt_idiv_const, 32);
       const nir_lower_idiv_options options = {
-         .imprecise_32bit_lowering = false,
          .allow_fp16 = false
       };
       OPT(nir_lower_idiv, &options);