cmd/compile: preserve 'any' type alias in unified IR
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 <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
Run-TryBot: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

golang/go,CAFxX/go,golang/go,cloudflare/go,nawawi/go,cloudflare/go,nawawi/go,nawawi/go,cloudflare/go,golang/go,golang/go,cloudflare/go,cloudflare/go,golang/go,golang/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,CAFxX/go,CAFxX/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,cloudflare/go,nawawi/go