[DAG] MergeInnerShuffle with BinOps - sometimes accept undef mask elements

If the inner shuffle already contains undef elements, then accept them in the merged shuffle as well.

This helps some X86 HADD/SUB patterns where slow targets were ending up with HADD/SUB because the (un)merged shuffles were stuck either side of the ADD/SUB - meaning we ended up with a total cost much higher than the "2*shuffle+add" that a slow target usually expands a HADD/SUB to.