[DAGCombiner] Peek through vector concats when trying to combine shuffles.

Summary: This combine showed up as needed when exploring the regression when processing the DAG in topological order.

Reviewers: craig.topper, efriedma, RKSimon, lebedev.ri

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68195