[x86] Fix a bug in the v8i16 shuffling exposed by the new splat-like
lowering for v16i8.

ASan and some bots caught this bug with existing test cases. Fixing it
even fixed a miscompile with one of the test cases. I'm still a bit
suspicious of this test case as I've not taken a proper amount of time
to think about it, but the fix here is strict goodness.