[SelectionDAGBuilder] Pass fast math flags to getNode calls rather than trying...

[SelectionDAGBuilder] Pass fast math flags to getNode calls rather than trying to set them after the fact.:

This removes the after the fact FMF handling from D46854 in favor of passing fast math flags to getNode. This should be a superset of D87130.

This required adding a SDNodeFlags to SelectionDAG::getSetCC.

Now we manage to contant fold some stuff undefs during the
initial getNode that we don't do in later DAG combines.

Differential Revision: https://reviews.llvm.org/D87200