[AVX512] Add masking variant and intrinsics for valignd/q

This is similar to what I did with the two-source permutation recently.  (It's
almost too similar so that we should consider generating the masking variants
with some tablegen help.)

Both encoding and intrinsic tests are added as well.  For the latter, this is
what the IR that the intrinsic test on the clang side generates.

Part of <rdar://problem/17688758>

llvm-svn: 214890