bogus warnings from constant-folding

[Nikodemus Siivola]

  The compiler sometimes tries to constant-fold expressions before
  it checks to see whether they can be reached. This can lead to
  bogus warnings about errors in the constant folding, e.g. in code
  like
    (WHEN X
      (WRITE-STRING (> X 0) "+" "0"))
  compiled in a context where the compiler can prove that X is NIL,
  and the compiler complains that (> X 0) causes a type error because
  NIL isn't a valid argument to #'>. Until sbcl-0.7.4.10 or so this
  caused a full WARNING, which made the bug really annoying because then
  COMPILE and COMPILE-FILE returned FAILURE-P=T for perfectly legal
  code. Since then the warning has been downgraded to STYLE-WARNING,
  so it's still a bug but at least it's a little less annoying.

[Stas Boukarev (stassats)]

Can no longer reproduce.