[InstCombine] propagate FMF for fcmp+fabs folds

By morphing the instruction rather than deleting and creating a new one,
we retain fast-math-flags and potentially other metadata (profile info?).


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346331 91177308-0d34-0410-b5e6-96231b3b80d8