cmd/compile: pair loads and stores on arm64

cmd/compile: pair loads and stores on arm64

Look for possible paired load/store operations on arm64.
I don't expect this would be a lot faster, but it will save
binary space, and indirectly through the icache at least a bit
of time.

Change-Id: I4dd73b0e6329c4659b7453998f9b75320fcf380b
Reviewed-on: https://go-review.googlesource.com/c/go/+/629256
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>