[dev.simd] simd, cmd/compile: add more element types for Select128FromPair

[dev.simd] simd, cmd/compile: add more element types for Select128FromPair

Also includes a comment cleanup pass.
Fixed NAME processing for additional documentation.

Change-Id: Ide5b60c17ddbf3c6eafd20147981c59493fc8133
Reviewed-on: https://go-review.googlesource.com/c/go/+/722180
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>