compare on exceptional Doubles and Floats should raise an error

[igloo]

Consider:

let n = 0/0 :: Double in (n `compare` n, n < n, n == n, n > n)

In GHC and YHC this gives

(GT,False,False,False)

while in hugs it gives

(EQ,False,False,False)

Neither of these is very satisfactory, as I would expect

x `compare` y === EQ   =>   (x == y) === True
x `compare` y === GT   =>   (x > y) === True

and it's even less pleasant that the implementations differ for no good reason.

The Haskell report isn't very helpful on how comparing exceptional Doubles should behave, as it doesn't even say you need to have NaN etc:

http://haskell.org/onlinereport/basic.html#sect6.4
The results of exceptional conditions (such as overflow or 
underflow) on the fixed-precision numeric types are undefined; an
implementation may choose error (_|_, semantically), a truncated
value, or a special value such as infinity, indefinite, etc.

I think that the right answer is that

n `compare` n

above (and more generally such a comparison for any incomparable Doubles or Flaots) should raise an error (i.e. be _|_).

The changes needed are simple, e.g. for GHC

(D# x) `compare` (D# y) | x <## y   = LT
                        | x ==## y  = EQ
                        | otherwise = GT

becomes

(D# x) `compare` (D# y) | x <## y   = LT
                        | x ==## y  = EQ
                        | x >## y   = GT
                        | otherwise = error "Incomparable values"

Deadline: 1 week after discussion ends.

[igloo]

Moved to Haskell':  http://hackage.haskell.org/trac/haskell-prime/ticket/123