mie: add parent pointers to region and block; two-way link between a register and its users

mie/ir/op-arg.c

@@ -3,7 +3,7 @@
 const struct mie_type *mie_op_arg_get_type(const struct mie_op_arg *arg)
 {
 	if (arg->arg_flags & MIE_OP_F_ARG_RESOLVED) {
-		return arg->arg_value->reg_type;
+		return arg->arg_value.u_reg->reg_type;
 	}
 
 	return arg->arg_unresolved.reg_type;

mie/ir/region.c

@@ -5,5 +5,14 @@
 
 struct mie_block *mie_region_add_block(struct mie_region *region)
 {
-	return mie_vector_emplace_back(region->r_blocks, &mie_block_vector_ops);
+	struct mie_block *block = mie_vector_emplace_back(
+		region->r_blocks, &mie_block_vector_ops);
+
+	if (!block) {
+		return NULL;
+	}
+
+	block->b_parent = region;
+
+	return block;
 }