[DAG] propagate FMF for all FPMathOperators

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.

Differential Revision: https://reviews.llvm.org/D46854

llvm-svn: 332358

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