ir_reader: Relax requirement that function arguments be s_lists.
ir_reader: Relax requirement that function arguments be s_lists.

All of these functions used to take s_list pointers so they wouldn't all
need SX_AS_LIST conversions and error checking.  However, the new
pattern matcher conveniently does this for us in one centralized place.

So there's no need to insist on s_list.  Switching to s_expression saves
a bit of code and is somewhat cleaner.

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