cmd/compile: use OpMove instead of memmove more on arm64

cmd/compile: use OpMove instead of memmove more on arm64

OpMove is faster for small moves of fixed size.

For safety, we have to rewrite the Move rewrite rules a bit so that
all the loads are done before any stores happen.

Also use an 8-byte move instead of a 16-byte move if the tail is
at most 8 bytes.

Change-Id: I7f6c7496ac6d5eb2e0706fd59ca4b5d797c51101
Reviewed-on: https://go-review.googlesource.com/c/go/+/672997
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>