[dev.simd] cmd/compile, simd: add VPALIGNR

This CL named VPALIGNR ConcatShiftBytes[Grouped].

Change-Id: I46c6703085efb0613deefa512de9911b4fdf6bc4
Reviewed-on: https://go-review.googlesource.com/c/go/+/714440
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>