cmd/compile: add missing shape check in (*Tsubster).tinter

Add a missing shape check in (*Tsubster).tinter when substituting on a
generic type which is an empty interface, analogous to same check in
(*Tsubster).tstruct. Empty structs/interfaces that have rparams (i.e.
are a generic type or a shape type) need to get a new type of their
rparams - they will be different even though they don't have any
fields/methods. Without this shape check, we were not correctly
completing the Token[int] type during substitution in the example in the
issue. This issue only happens for a generic type which is an empty
interface (i.e. doesn't actually use the type param, hence quite unusual).

Added the test case already created by Keith.

Fixes #50841

Change-Id: Ia985b9f52c0e87ed0647b46373e44c51cb748ba4
Reviewed-on: https://go-review.googlesource.com/c/go/+/381175


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