preliminary patch for removing more NEGATE in fold-const

[Sylvain Pion]

Hi,

Here is a preliminary patch (because not fully tested yet) that enables some
negate "absortions" in fold-const.c::fold().  Most of them apply to IEEE FP,
but some have been enabled for fast-math, and some for integers too.

If you could just check that I'm not completely mistaken, I could go one and
polish the testing before final submission.  The patch is against 19990405.

Thanks,
-- 
Sylvain