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Instructions that produce more than one result complicate
optimizations. get_list/3 is one of two instructions that
produce multiple results (get_map_elements/3 is the other).
Introduce the get_hd/2 and get_tl/2 instructions
that return the head and tail of a cons cell, respectively,
and use it internally in all optimization passes.
For efficiency, we still want to use get_list/3 if both
head and tail are used, so we will translate matching pairs
of get_hd and get_tl back to get_list instructions.
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