[InstCombine] add folds for fcmp Pred fabs(X), 0.0

Similar to rL346321, we had folds for the ordered
versions of these compares already, so add the
unordered siblings for completeness.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346324 91177308-0d34-0410-b5e6-96231b3b80d8