cmd/compile: don't expand invalid embedded interface

cmd/compile: don't expand invalid embedded interface

The invalid interface type will be reported already, so don't expand
that invalid one, which causes the compiler crashes.

Updates #43311

Change-Id: Ic335cfa74f0b9fcfd0929dc5fd31d9156a8f5f5c
Reviewed-on: https://go-review.googlesource.com/c/go/+/298710
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>