cmd/compile: correct type identity comparison with "any"

The builtin "any" type should only be identical to an unnamed empty
interface type, not a defined empty interface type.

Fixes #50169.

Change-Id: Ie5bb88868497cb795de1fd0276133ba9812edfe4
Reviewed-on: https://go-review.googlesource.com/c/go/+/372217


Trust: Cherry Mui <cherryyz@google.com>
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Dan Scales <danscales@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>