cmd/compile: preserve 'any' type alias in unified IR

When exporting the "any" empty interface type for unified IR, write it
out as a reference to the "any" alias, rather than to the underlying
empty interface. This matches how "byte" and "rune" are handled.

Verified to fix the issue demonstrated in CL 369975.

Change-Id: Ic2844b0acc3b17c20b3a40aaf262f62ec653eb5a
Reviewed-on: https://go-review.googlesource.com/c/go/+/370374


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>