These three trivial data-movement handlers are a useful first
migration target -- they exercise the new `temp` declaration syntax
and the register allocator end-to-end without interacting with macros
or fixed-operand constraints. The allocator picks t1-equivalent
registers for the first temp (which is the smallest live range that
must survive load_operand's rax/x9 scratch use) and reuses the slot
across non-overlapping live ranges where possible.
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