cmd/compile: fix stenciling of conversions between interfaces

Conversions need to allow nil through.
We do that using a CONVIFACE instead of a DOTTYPE.

Also reorganize how nonempty interfaces are handled.
For nonempty to empty, a simple CONVIFACE suffices.
For nonempty to nonempty, we need to call the runtime to get the
new itab. Use the entry from the dictionary to identify the
target interface type (if parameterized).

Fixes #47925

Change-Id: I4ffeed964318bb3e270b06f558e6ab9c5bfc7188
Reviewed-on: https://go-review.googlesource.com/c/go/+/344830


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