[X86] Add back AVX2 VR256 PMOVX patterns.
[X86] Add back AVX2 VR256 PMOVX patterns.

We can't reach those from zext, but other parts of the backend (the shuffle
lowering) generate 256-bit VZEXT nodes.

Fixes PR21876.


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@223996 91177308-0d34-0410-b5e6-96231b3b80d8

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