func (po *poset) upushalias(id ID, i2 uint32) {
	po.undo = append(po.undo, posetUndo{typ: undoAliasNode, ID: id, idx: i2})
}

// upushconst pushes a new undo pass for a new constant
func (po *poset) upushconst(idx uint32, old uint32) {
	po.undo = append(po.undo, posetUndo{typ: undoNewConstant, idx: idx, ID: ID(old)})
}

// addchild adds i2 as direct child of i1.