cmd/compile: fix recursive generic interface instantiation

When shapifying recursive instantiated types, the compiler ends up
leaving the type as-is if it already has been a shape type. However, if
both of type arguments are interfaces, and one of them is a recursive
one, it ends up being shaped as-is, while the other is shaped to its
underlying, causing mismatch in function signature.

Fixing this by shapifying an interface type as-is, if it is fully
instantiated and already been a shape type.

Fixes #65362
Fixes #66663

Change-Id: I839d266e0443b41238b1b7362aca09adc0177362
Reviewed-on: https://go-review.googlesource.com/c/go/+/559656


Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>