[x86] Fully generalize the zext lowering in the new vector shuffle
lowering to support both anyext and zext and to custom lower for many
different microarchitectures.

Using this allows us to get *exactly* the right code for zext and anyext
shuffles in all the vector sizes. For v16i8, the improvement is *huge*.
The new SSE2 test case added I refused to add before this because it was
sooooo muny instructions.