cmd/compile: deduplicate OpArg's across types

For in-register arguments, it must have only a single copy of it
present in the function. If there are multiple copies, it confuses
the register allocator, as they are in the same register.

Change-Id: I55cb06746f08aa7c9168026d0f411bce0a9f93f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/306330


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