bug RSSA backend

[Matthew Fluet]

> I think something is messed up in
> MLton.Random.alphaNumString, because it seems to always return EEEEEE.

Turns out this is really a bug in the SSA restore pass, which has already
been fixed on the main branch.

I think we can merge in the various non-backend related changes into the
rssa branch via  cvs update -j BASE [files]...  ; although, I wouldn't do
so in the backend and/or codegen directories.  It shouldn't harm anything,
other than reducing the number of conflicts when we join back in the other
direction.