fix misoptimization of TRUNCATE

fix misoptimization of TRUNCATE

  Reported by Eric Marsden on sbcl-devel 2011-12-01.

   "illegal instruction on PowerPC"

  We check for result type being a VALUES-TYPE-P when deciding if to compute
  the second value for TRUNCATE or not -- but *WILD-TYPE* isn't a values type.

  Make VALUES-TYPE-P return true for it from now on. What could possibly go
  wrong? Just two other places need to change, it seems.