[AVX-512] Unpack support in new shuffle lowering

[Adam Nemet]

The change should be obvious.  For the test, I decided to start resurrecting
(and extending) avx512-shuffle.ll.  This was removed when the new shuffle
lowering was made default.

We could also add these to vector-shuffle-512-v8.ll but I think those are
somehow generated with Chandler's fuzzer, so I decided against it.

Chandler, is this reasonable?

Thanks,
Adam

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