[GISel][CombinerHelper] Add a combine turning shuffle_vector into concat_vectors

Teach the CombinerHelper how to turn shuffle_vectors, that
concatenate vectors, into concat_vectors and add this combine
to the AArch64 pre-legalizer combiner.

Differential Revision: https://reviews.llvm.org/D69149

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375452 91177308-0d34-0410-b5e6-96231b3b80d8