cmd/compile: remove self copies in tail-call wrappers

The previous CL re-enables tail calls for method wrappers. But
with the changed IR and SSA representation, for stack arguments
it generates self copies. This CL makes the compiler detect the
self copies and remove them.

Change-Id: I7252572a1a47834f28b6706e45906e2356408e02
Reviewed-on: https://go-review.googlesource.com/c/go/+/350349


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