cmd/compile: fix CONVIFACE case converting interface to empty interface

We need an extra case in convertToDictionary. In the case of an operand
which is an interface and converting to an empty interface, we don't
want to get the run-time type from the dictionary (which would be the
run-time type of the interface). We want to do a type-assert to the
empty interface.

Change-Id: I414247210168153151272fab198bfe82ad7b1567
Reviewed-on: https://go-review.googlesource.com/c/go/+/342009


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