[X86][SSE] Attempt to fold shuffle(binop(),binop()) -> binop(shuffle(),shuffle())

If this will help us fold shuffles together, then push the shuffle through the merged binops.

Ideally this would be performed in DAGCombiner::visitVECTOR_SHUFFLE but getting an efficient+legal merged shuffle can be tricky - on SSE we can be confident that for 32/64-bit elements vectors shuffles should easily fold.