[GISel][CombinerHelper] Add concat_vectors(build_vector, build_vector) => build_vector

Teach the combiner helper how to flatten concat_vectors of build_vectors
into a build_vector.

Add this combine as part of AArch64 pre-legalizer combiner.

Differential Revision: https://reviews.llvm.org/D69071

llvm-svn: 375066