[m-rev.] for possible post-commit review: eliminate unneeded work in arg_info.m

[Zoltan Somogyi]

On Fri, 01 Apr 2022 06:42:34 +1100 (AEDT), "Zoltan Somogyi" <zoltan.somogyi at runbox.com> wrote:
> This should not need review.

On my laptop, this diff speeds up speedtest in hlc.gc by 1%,
which I think is big enough to be measured reasonably reliably.

Zoltan.