cmd/compile: make write barrier code amenable to paired loads/stores

cmd/compile: make write barrier code amenable to paired loads/stores

It currently isn't because it does load/store/load/store/...
Rework to do overwrite processing in pairs so it is instead
load/load/store/store/...

Change-Id: If7be629bc4048da5f2386dafb8f05759b79e9e2b
Reviewed-on: https://go-review.googlesource.com/c/go/+/631495
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>