cmd/compile: fix transform.AssignOp to deal with tricky case

When going to dictionary formats derived from the function
instantiations, I had broken out noder.Assignop() to deal specially with
shape types, but didn't quite get the tricky case right. We still need
to allow conversion between shape types, but if the destination is an
interface, we need to use CONVIFACE rather than CONVNOP.

Fixes #48453.

Change-Id: I8c4b39c2e628172ac34f493f1dd682cbac1e55ae
Reviewed-on: https://go-review.googlesource.com/c/go/+/350949


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