[x86] Teach the unpack lowering how to lower with an initial unpack in

[x86] Teach the unpack lowering how to lower with an initial unpack in
addition to lowering to trees rooted in an unpack.

This saves shuffles and or registers in many various ways, lets us
handle another class of v4i32 shuffles pre SSE4.1 without domain
crosses, etc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229856 91177308-0d34-0410-b5e6-96231b3b80d8