[SDAG] simplify FP binops to undef

As discussed in the commit thread for rGa253a2a and D73978, we can do more undef folding for FP ops.
The nnan and ninf fast-math-flags specify that if an operand is the disallowed value, the result is
poison, so we can produce an undef result.

But this doesn't work as expected (the undef operand cases remain) because of a Flags propagation
problem in SelectionDAGBuilder.

I've added DAGCombiner calls to enable these for the other cases because we've shown in other
patches that (because of the limited way that SDAG iterates), it is possible to miss simplifications
like this if they are done only at node creation time.

Several potential follow-ups to expand on this patch are possible.

Differential Revision: https://reviews.llvm.org/D75576