[m-rev.] for review: Remove mentions of 'dead' and 'mostly_dead' from reference manual.

[Peter Wang]

On Sat,  5 Aug 2017 11:42:05 +1000, Peter Wang <novalazy at gmail.com> wrote:
> doc/reference_manual.texi:
>     Use 'clobbered' and 'mostly_clobbered' everywhere instead of the
>     synonyms 'dead' and 'mostly_dead'.

We could also remove the special case for "dead" and "mostly_dead" in
check_mutable_inst, seeing as the original reason for that was because
the reference manual referred to those insts by name.
It's not important, though.

Peter