[X86][SSE] Add support for combining scalar_to_vector(extract_vector_elt) into a target shuffle.

Correctly flagging upper elements as undef.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@294020 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed