Custom-expand legal VECREDUCE_FADD SDNodes
to benefit from pair-wise faddp instructions.
The accumulator argument is not part of VECREDUCE_FADD, and is only represented in the VECREDUCE_STRICT_FADD for the reason that it _must_ start off with that value as part of the ordered reduction, where for the non-strict VECREDUCE_FADD it can be accumulated separately using a regular FADD. There should probably be a separate SDT_FPStrictVecReduce when we implement this for VECREDUCE_STRICT_FADD.
Looking in SelectionDAGBuilder::visitVectorReduce() I see that the accumulator operand is actually ignored for non-strict fadd reductions, which is something that we'll need to fix.