[dev.simd] simd: make "best" instruction choice also depend on commutativity

the compare-based-on-immediate instructions are sometimes commutative,
sometimes not.  In this case, that means the instruction cannot be
commutative.

also improve the comments for comparisons.

Change-Id: I83a55fa5ffbd6cbbaf5cb23b3e8a68a5da8aae2f
Reviewed-on: https://go-review.googlesource.com/c/go/+/726440
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Austin Clements <austin@google.com>

Author: dr2chase