cmd/compile: support memmove inlining with register args

The rule that inlines memmove expects SSA ops that calls memmove
with arguments in memory. This CL adds a version that matches
it with arguments in registers, so the optimization works for
both situations.

Change-Id: Ideb64f65b7521481ab2ca7c9975a6cf7b70d5966
Reviewed-on: https://go-review.googlesource.com/c/go/+/309332


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: David Chase <drchase@google.com>