bug[next]: Fix ITIR nested tuple return tracing

[tehrengruber]

While returning a tuple from an ITIR stencil could be successfully traced

@fundef
def tuple_output1(inp1, inp2):
    return (deref(inp1), deref(inp2))

nested tuples were broken, which is fixed by this PR.

@fundef
def tuple_of_tuple_output1(inp1, inp2, inp3, inp4):
    return (deref(inp1), deref(inp2)), (deref(inp3), deref(inp4))