term goal here is to be able to match enough of vector_shuffle and build_vector
so all avx intrinsics which aren't mapped to their own built-ins but to
shufflevector calls can be codegen'd. This is the first (baby) step, support
building zeroed vectors.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@110897 91177308-0d34-0410-b5e6-96231b3b80d8
