[dev.simd] cmd/compile: updates simd ordering and docs

[dev.simd] cmd/compile: updates simd ordering and docs

This CL is generated by CL 681395.

Change-Id: Ic930aeeb24fc7f95a4d74c77403532d0b0eb39ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/681215
Auto-Submit: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>