simd/archsimd: add Grouped for 256- and 512-bit SaturateTo(U)Int16Concat, and fix type
They operate on 128-bit groups, so name them Grouped to be clear,
and consistent with other grouped operations. Reword the
documentation, mention the grouping only for grouped versions.
Also, SaturateToUnt16Concat(Grouped) is a signed int32 to unsigned
uint16 saturated conversion. The receiver and the parameter should
be signed. The result remains unsigned.
Change-Id: I30e28bc05e07f5c28214c9c6d9d201cbbb183468
Reviewed-on: https://go-review.googlesource.com/c/go/+/731501 Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>