intel/compiler: Fold constants after distributing source modifiers

This can generate things like fneg! of load_const, which is silly.
Fold those away into an actual constant.  Only do so on the scalar
backend because there's a comment above that the vec4 backend doesn't
want any new constants this late, and I'm inclined to believe it.

fossil-db stats show a very minor improvement:

   Totals:
   Instrs: 203091223 -> 203091099 (-0.00%); split: -0.00%, +0.00%
   Cycles: 14410638075 -> 14410577067 (-0.00%); split: -0.00%, +0.00%

   Totals from 20 (0.00% of 665070) affected shaders:
   Instrs: 27067 -> 26943 (-0.46%); split: -0.47%, +0.01%
   Cycles: 2687958 -> 2626950 (-2.27%); split: -2.27%, +0.00%

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22881>

src/intel/compiler/brw_nir.c

@@ -1639,6 +1639,9 @@ brw_postprocess_nir(nir_shader *nir, const struct brw_compiler *compiler,
       OPT(nir_lower_alu_to_scalar, NULL, NULL);
 
    while (OPT(nir_opt_algebraic_distribute_src_mods)) {
+      if (is_scalar)
+         OPT(nir_opt_constant_folding);
+
       OPT(nir_copy_prop);
       OPT(nir_opt_dce);
       OPT(nir_opt_cse);