[InstCombine] add fold for fabs(X) u< 0.0

The sibling fold for 'oge' --> 'ord' was already here,
but this half was missing. 

The result of fabs() must be positive or nan, so asking 
if the result is negative or nan is the same as asking 
if the result is nan.

This is another step towards fixing:
https://bugs.llvm.org/show_bug.cgi?id=39475


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346321 91177308-0d34-0410-b5e6-96231b3b80d8