[dev.typeparams] cmd/compile: handle derived types that are converted to interfaces

Up to this point, we were only handling typeparams that were converted
to empty or non-empty interfaces. But we have a dictionary entry for
each derived type (i.e. type derived from typeparams) as well. So, when
doing a conversion, look for the source type in both the type params and
derived types of the generic info, and then use the appropriate
dictionary entry.

Added some cases to ifaceconv.go (e.g. converting []T to an empty
interface).

Change-Id: I7bbad0128bec20ccccd93ae1d65c1ffd44ca79a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/333011


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