[x86] Actually test the SSE2 lowering for most of the zext-ish shuffles.

Not sure why I only did SSSE3 here. Also, I've left out some of the SSE2
ones because the shuffles are so absurd it's not worth transcribing
them. Will try to fix them to be sane and then check them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218114 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Recognize that we can use duplication to widen v16i8 shuffles due
to undef lanes as well as defined widenable lanes. This dramatically
improves the lowering we use for undef-shuffles in a zext-ish pattern
for SSE2.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218115 91177308-0d34-0410-b5e6-96231b3b80d8