This register reuse here didn't go through our locations-tracking
infrastructure, and it just assumed it'd be ok to do.  I think it's
only called from places where the register we reuse won't be used
after this function.

This broke though when I added the ability to allocate orig-arg
registers; now, the argument is still dead in that it won't be used
by the function, but it could be "used" if a guard failed.  But this
wasn't being tracked in the locations map, so we ended up trashing
the orig arg despite the lengths we went to to not do that :/