nir/opt_deref: Add an optimization for deref_mode_is

If opt_restrict_deref_modes makes progress, we may be able to figure out
the mode well enough to turn a deref_mode_is intrinsic into a constant.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6332>

src/compiler/nir/nir_deref.c

@@ -1268,6 +1268,30 @@ opt_store_vec_deref(nir_builder *b, nir_intrinsic_instr *store)
    return false;
 }
 
+static bool
+opt_known_deref_mode_is(nir_builder *b, nir_intrinsic_instr *intrin)
+{
+   nir_variable_mode modes = nir_intrinsic_memory_modes(intrin);
+   nir_deref_instr *deref = nir_src_as_deref(intrin->src[0]);
+   if (deref == NULL)
+      return false;
+
+   nir_ssa_def *deref_is = NULL;
+
+   if (nir_deref_mode_must_be(deref, modes))
+      deref_is = nir_imm_true(b);
+
+   if (!nir_deref_mode_may_be(deref, modes))
+      deref_is = nir_imm_false(b);
+
+   if (deref_is == NULL)
+      return false;
+
+   nir_ssa_def_rewrite_uses(&intrin->dest.ssa, nir_src_for_ssa(deref_is));
+   nir_instr_remove(&intrin->instr);
+   return true;
+}
+
 bool
 nir_opt_deref_impl(nir_function_impl *impl)
 {
@@ -1318,6 +1342,11 @@ nir_opt_deref_impl(nir_function_impl *impl)
                   progress = true;
                break;
 
+            case nir_intrinsic_deref_mode_is:
+               if (opt_known_deref_mode_is(&b, intrin))
+                  progress = true;
+               break;
+
             default:
                /* Do nothing */
                break;