cmd/compile: mark PCMPEQB as commutative

cmd/compile: mark PCMPEQB as commutative

compilecmp linux/amd64:

internal/runtime/maps
internal/runtime/maps.(*table).Delete changed

internal/runtime/maps [cmd/compile]
internal/runtime/maps.(*Map).Delete changed
internal/runtime/maps.(*table).Delete changed

Change-Id: Ic3c95411c23cab7427e63105170de41e5766f809
Reviewed-on: https://go-review.googlesource.com/c/go/+/647935
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@golang.org>