Each instruction automatically records the tree_instr node to
which it is attached. The parent method retrieves this pointer.
In an expression tree, all of the instructions share the same parent
tree_instr node, so an instruction may not be directly connected
to its parent node.
The owner method is identical to the parent method except
for instructions that are used as operands of tree_for nodes.
Those operands are actually attached to dummy copy instructions on lists
attached to the tree_for nodes (see section For Nodes), but many
users will want to treat them as if they are directly attached to the
tree_for nodes. Thus, for an instruction in a tree_for
operand, the owner method will return a pointer to the
tree_for node instead of returning the actual parent
tree_instr. This only works if the tree_for operand list
contains a single expression (not a flat list); otherwise, the actual
tree_instr parent is returned.