[AVX-512] Add tests for masked palignr/valignd/valignq shuffles, many of which show...

[AVX-512] Add tests for masked palignr/valignd/valignq shuffles, many of which show failures to fold the masking into the operation.

Many of these problems are because shuffle lowering widens element size and reduces element count when possible. This causes the shuffle to become separated from the select by a bitcast. Future patches will work to improve these cases by rewriting the shuffle back to a narrow element type if we think it can result in folding the mask.

llvm-svn: 287503