cmd/compile: canonicalize empty interface types

cmd/compile: canonicalize empty interface types

Mapping all empty interfaces onto the same Type
allows better reuse of the ptrTo and sliceOf
Type caches for *interface{} and []interface{}.

This has little compiler performance impact now,
but it will be helpful in the future,
when we will eagerly populate some of those caches.

Passes toolstash-check.

Change-Id: I17daee599a129b0b2f5f3025c1be43d569d6782c
Reviewed-on: https://go-review.googlesource.com/38344
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>