cmd/compile: preserve name association when eliding copies in expand_calls

If v is a Copy of x, we will rewrite v to x. If v has a name
associated to it, let the name associate to x.

Under register ABI, this helps associate in-register Arg values
to the parameters' names. (But does not address all cases.)

Change-Id: I47c779e56c9d0823a88890497e32326bc0290f82
Reviewed-on: https://go-review.googlesource.com/c/go/+/309330


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>
Reviewed-by: Than McIntosh <thanm@google.com>