[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375066 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed