Fixed a bug when lowering build_vector (PR19694)

When lowering build_vector to an insertps, we would still lower it, even
if the source vectors weren't v4x32. This would break on avx if the source
was a v8x32. We now check the type of the source vectors.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208487 91177308-0d34-0410-b5e6-96231b3b80d8