[x86] Add some tests for a common unpack pattern of vector shuffle that
has a remarkably unique and efficient lowering.

While we get this some of the time already, we miss a few cases and
there wasn't a principled reason we got it. We should at least test
this. v8 already has tests for this pattern.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222607 91177308-0d34-0410-b5e6-96231b3b80d8