Word{8,32}.neg

[Matthew Fluet]

I sould also mention that this certainly isn't a crucial optimization.  In
all of the regression and benchmark programs,  filesys.sml  would benefit
the most, and in that case, there are only 6 opportunities to make the
optimization.  (A self compile might have a few more, but I don't have
that number right now.)

The other interesting note is that it never occured in the Word8 case, but
if we added one, the other one would be trivial to add in as well.