Folding constants for floats

[Sven Panne]

... and let's not forget about such fun stuff as IEEE's -0, e.g.:

   1/(-1 * 0)   =>   -Infinity
   1/(0 + (-1 * 0))   =>   Infinity

If we take the standpoint that Haskell's Float and Double types
correspond to IEEE 754 floating point numbers, there is almost no
mathematical equivalence which holds, and consequently almost all
folding or other optimizations will be wrong. One can do all these
things behind a flag (trading IEEE compatibility for better code), but
this shouldn't be done by default IMHO.