[DAG] Fold shuffle(bop(shuffle(x,y),shuffle(z,w)),undef) -> bop(shuffle'(x,y),shuffle'(z,w))

Followup to D96345, handle unary shuffles of binops (as well as binary shuffles) if we can merge the shuffle with inner operand shuffles.

Differential Revision: https://reviews.llvm.org/D98646