cmd/compile: fix implement for closure in a global assignment

If closure in a global assignment and has a method receiver.
We should assign receiver as a global variable, not a local variable.

Fixes #48225

Change-Id: I8f65dd6e8baf66a5eff24028d28ad0a594091add
Reviewed-on: https://go-review.googlesource.com/c/go/+/348512


Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>