Add support for combining a build vector to a shuffle.
When the build vector is of extracted elements from 2 vectors (vec1, vec2) where vec2 is 2 times smaller than vec1.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305883 91177308-0d34-0410-b5e6-96231b3b80d8