[dev.simd] simd, cmd/compile: add 128 bit select-from-pair

[dev.simd] simd, cmd/compile: add 128 bit select-from-pair

Using this name until a better one appears:
   x.Select128FromPair(3, 2, y)

Includes test for constant and variable case.
Checks for unexpected immediates (using the zeroing flag,
which is not supported for this intrinsic) and panics.

Change-Id: I9249475d6572968c127b4ee9e00328d717c07578
Reviewed-on: https://go-review.googlesource.com/c/go/+/705496
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>