For register-register move, if there is only one use, allocate it in
the same register so we don't need to emit an instruction.

Updates #15365.

Change-Id: Iad41843854a506c521d577ad93fcbe73e8de8065
Reviewed-on: https://go-review.googlesource.com/25059
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>