When assembling raw instructions, often you want to or together several
fields for clarity, or because you're using an assembly macro with
separate arguments.
This brings the use of `.insn` closer into line with what can be done
with the binutils assembler.
The 64-bit instruction test here explicitly sets the top bit to make
sure this works, even though the assembler is really using `int64_t`
in most places internally.
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