[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: https://llvm.org/svn/llvm-project/llvm/trunk@223996 91177308-0d34-0410-b5e6-96231b3b80d8