[dev.typeparams] cmd/compile: handle dictionaries for top-level instantiations

There's no outer function in these cases, so we won't be reading
the dictionary as a subdictionary from the outer scope's dictionary.
It will always be a compile-time constant.

Change-Id: I754b126652a6ffb62255734d53fcec29d77cfa9e
Reviewed-on: https://go-review.googlesource.com/c/go/+/324949


Trust: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Dan Scales <danscales@google.com>