This is a simple hack based on what's proposed in D37686, but we can extend it if needed in follow-ups. It gets us most of the FMF functionality that we want without adding any state bits to the flags. It also intentionally leaves out non-FMF flags (nsw, etc) to minimize the patch.
It should provide a superset of the functionality from D46563 - the extra tests show propagation and codegen diffs for fcmp, vecreduce, and FP libcalls.
The PPC log2() test shows the limits of this most basic approach - we only applied 'afn' to the last node created for the call. AFAIK, there aren't any libcall optimizations based on the flags currently, so that shouldn't make any difference.