small perf tweaks

clearing reg_rrr and reg_mmm more efficiently is an extremely small win,
but a win

read_imm_signed generally should inline well and runs afoul of some
heuristic. inlining gets about 8% improved throughput on the
(unrealistic) in-repo benchmark

it would be great to be able to avoid bounds checks somehow; it looks
like they alone are another ~10% of decode time. i'm not sure how to
pull that off while retaining the generic iterator parameter. might just
not be possible.

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