[dev.simd] cmd/compile, simd: add 64-bit select-from-pair methods

[dev.simd] cmd/compile, simd: add 64-bit select-from-pair methods

these are in the same style as the 32-bit select-from-pair,
including the grouped variant.  This does not quite capture
the full awesome power of VSHUFPD where it can select
differently in each group; that will be some other method,
that is more complex.

Change-Id: I807ddd7c1256103b5b0d7c5d60bd70b185e3aaf0
Reviewed-on: https://go-review.googlesource.com/c/go/+/705695
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>