nir/deref: Add an alu-of-cast optimization

Casts shouldn't change the bit pattern of the deref and you have to cast
again after you're done with the ALU anyway so we can ignore casts on
ALU sources.  This means we can actually start constant folding NULL
checks even if there are annoying casts in the way.

Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15673>

src/compiler/nir/nir_deref.c

@@ -851,6 +851,30 @@ nir_deref_instr_fixup_child_types(nir_deref_instr *parent)
    }
 }
 
+static bool
+opt_alu_of_cast(nir_alu_instr *alu)
+{
+   bool progress = false;
+
+   for (unsigned i = 0; i < nir_op_infos[alu->op].num_inputs; i++) {
+      assert(alu->src[i].src.is_ssa);
+      nir_instr *src_instr = alu->src[i].src.ssa->parent_instr;
+      if (src_instr->type != nir_instr_type_deref)
+         continue;
+
+      nir_deref_instr *src_deref = nir_instr_as_deref(src_instr);
+      if (src_deref->deref_type != nir_deref_type_cast)
+         continue;
+
+      assert(src_deref->parent.is_ssa);
+      nir_instr_rewrite_src_ssa(&alu->instr, &alu->src[i].src,
+                                src_deref->parent.ssa);
+      progress = true;
+   }
+
+   return progress;
+}
+
 static bool
 is_trivial_array_deref_cast(nir_deref_instr *cast)
 {
@@ -1347,6 +1371,13 @@ nir_opt_deref_impl(nir_function_impl *impl)
          b.cursor = nir_before_instr(instr);
 
          switch (instr->type) {
+         case nir_instr_type_alu: {
+            nir_alu_instr *alu = nir_instr_as_alu(instr);
+            if (opt_alu_of_cast(alu))
+               progress = true;
+            break;
+         }
+
          case nir_instr_type_deref: {
             nir_deref_instr *deref = nir_instr_as_deref(instr);