[dev.simd] cmd/compile: remove VPADDD4

[dev.simd] cmd/compile: remove VPADDD4

It is from my sample SIMD compilation, not used in the real thing.
The actual operation is VPADDD128.

Also clean up some of my XXX comments.

Change-Id: Ic20a9dd3c8531e25d88ba045ccef70cb856790d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/698475
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>