[X86] Promote v8i1 shuffles to v8i32 instead of v8i64 if we have VLX.

We should have equally good shuffle options for v8i32 with VLX. This was spotted during my attempts to remove 512-bit vectors from SKX.

We still use 512-bits for v16i1, v32i1, and v64i1. I'm less sure we can handle those well with narrower vectors. i32 and i64 element sizes get the best shuffle support.

llvm-svn: 321291