Store just the first and last addresses of rep string loops

Splitting out from #2001 to make it easier to track.

This is another step in address elision for values that can be
reconstructed in post-processing. For rep string loops, rather than
recording the address in each iteration, we would record just the first and
the last, or the first and direction and size, with special handling for
interruptions via fault or signal. This will take extra branches and maybe
state in a TLS slot or something, but that should be worthwhile to reduce i/o.