mips64: improve code generated for 64->32-bit truncation

[Eric Christopher]

> trouble of investigating the less immediate improvement.  Ok to
> install?
> 
> ----
> 

A similar and more comprehensive patch was just added by Richard
Sandiford to the rewrite branch last week so we'll just go ahead with
that when I merge it back (soon).

Thanks though. :)

-eric

-- 
o/~ got caught stealing fire from the sky o/~